Our publications

Tracking Down the Broadband Polarimetric Properties of PG 1553+113

The Astrophysical Journal · 2026

We report on a nine-month monitoring campaign of the blazar PG 1553+113, relying on three observations carried out in 2025 with the Imaging X-ray Polarimetry Explorer (IXPE) and supported by multiwavelength facilities. The source displayed pronounced variability across the electromagnetic spectrum, with X-ray flux changes by up to a factor of ∼5 and complex evolution of the optical polarization properties, including one of the largest (exceeding 150°) and fastest rotations in the electric vector position angle (EVPA) ever recorded. This swing of the EVPA was also accompanied by a temporary drop of the optical polarization degree to nearly zero. Significant X-ray polarization was observed during the third IXPE pointing, with a polarization degree Π<SUB>X</SUB> = (18.4 ± 5.8)% and Ψ<SUB>X</SUB> = 74° ± 9° in the 2─8 keV band, while only upper limits were obtained in the first two epochs. The optical data show that the second IXPE observation occurred shortly after a dramatic optical polarization event characterized by a rapid EVPA swing and strong depolarization. Two possible scenarios may explain the broadband polarimetric behavior: (i) the superposition of two emitting regions with nearly orthogonal magnetic field configurations and variable relative contributions, and (ii) the interaction of a single emitting region with a shock that temporarily reorders the magnetic field. In both cases, the data support a picture in which the X-ray and optical emissions arise from closely related but not strictly cospatial regions within a dynamically evolving, magnetically structured jet.

Detection of an Optical Quasiperiodic Oscillation in the Blazar 3C 454.3

The Astrophysical Journal · 2026

We analyzed 19 yr of R-band data of the blazar 3C 454.3 from the Whole Earth Blazar Telescope archive, along with new data from its members and from public archives such as those provided by the Small and Moderate Aperture Research Telescope System and the Steward Observatory projects to search for quasiperiodic oscillations (QPOs). We detected a QPO of ∼433 days using Lomb─Scargle periodogram, which lasted from MJD 54980─58450 as detected by the weighted wavelet Z-transform technique, making it one of the most persistent QPOs ever detected in the optical regime. The phase dispersion minimization technique was also performed to further validate this QPO claim. We detected this signal at a global significance of 2.53σ across all methodologies. To explain the observed QPO, we have considered both models focused on the accretion disk around a supermassive black hole (SMBH), and those based purely on jet emissions. Plausible jet-based models involve a shock moving down the jet in a helical magnetic field, whereas the SMBH models could involve Lense─Thirring-effect-induced jet precession or dual jets in a binary SMBH system. We introduce a novel approach to distinguish genuine QPOs from spurious signals arising from annual seasonal gaps, a common limitation of ground-based observations.

Evidence for protostellar jets as a population of hadronic gamma-ray sources

Nature Astronomy · 2026

Stars are born in darkness, deep within cold, dense molecular clouds where gravity drives the collapse of gas and dust, which gives rise to protostars, the earliest stages of stellar evolution. Once considered purely thermal sources, these young systems are now emerging as sites of energetic non-thermal activity. Although radio synchrotron jets hint at the presence of relativistic electrons, direct confirmation of proton acceleration has remained elusive. Here we report a statistically significant detection of γ-rays from a population of young stellar objects, thus revealing a Galactic class of Gamma-Loud Protostars. Observations point towards particle acceleration within protostellar jets, where γ-ray emission arises from protons interacting with surrounding molecular clouds via pion decay. We find a correlation between cosmic-ray output and bolometric luminosity, which indicates that particle acceleration scales with the mechanical power of the system. These findings open a new observational window into the role of non-thermal processes in protostellar evolution and indicate that γ-ray studies of protostars could provide critical insights into accretion, ejection and feedback in star formation. This previously overlooked emission traces the energetic feedback that young stars inject into their surroundings, which shapes the conditions for subsequent star and planet formation.

Chasing Gamma-Ray Signals from Binary Neutron Star Coalescences with the Cherenkov Telescope Array: Prospects and Observing Strategies

The Astrophysical Journal · 2026

The detection of gravitational waves (GWs) from a binary neutron star (BNS) merger by Advanced LIGO and Advanced Virgo (GW170817), together with its electromagnetic counterpart, the short gamma-ray burst GRB 170817A, heralded the birth of multimessenger astronomy. The detection of TeV emission from GRBs motivates follow-up observations with the Cherenkov Telescope Array Observatory (CTAO), which is ideal for detecting such signals due to its unprecedented sensitivity, rapid response, and wide-field survey capabilities. The aim of this work is to evaluate GeV─TeV GW follow-up strategies for CTAO using a multistep simulation pipeline and to estimate the expected rate of joint GW─GRB detections during observing run O5. Using a simulated sample of BNS systems with corresponding GW detections, gamma-ray emission is simulated through phenomenological prescriptions based on the observed population of short GRBs, including off-axis jet scenarios. CTAO observations are simulated to account for instrument response, sky tiling strategies, integration times, and varying observing conditions. Strategies with variable and constant integration times are investigated. We find that, via an optimized follow-up strategy, about 5% of simulated GW-associated short GRBs produce GeV─TeV radiation detectable by CTAO. Detectability is strongly influenced by the jet opening angle and viewing angle, suggesting that even rough estimates of the viewing angle in GW alerts could enhance targeting. This framework motivates future follow-ups of GW-detectable events, including neutron star─black hole mergers, and further supports the development of advanced strategies incorporating galaxy distributions and synergies with future detectors such as the Einstein Telescope.

The January 2010 flare of Mrk 421: Insights from a stochastic acceleration model

Astronomy and Astrophysics · 2026

Context. Mrk 421 displayed its highest flux state ever observed in February 2010 with very high tera-electronvolt fluxes and interesting cross-band correlations and a spectral energy distribution (SED) evolution not entirely consistent with the standard single zone leptonic synchrotron self-Compton model. The source was already in a high state in January 2010 and displayed strong variability in the days preceding the highest state. We study the temporal evolution of the spectra in January to extract information about the particle dynamics and the physical properties of the emission region. Aims. We build on the temporal variability and correlations studied in our previous work and attempt to improve the SED model fits with a physics-oriented approach. Methods. The The multi-wavelength data were processed and the SEDs were fit using JetSeT. The SED evolution and cross-band correlations were modelled using leptonic log-parabola with a low energy power-law branch (LPPL) and pile-up distributions that are predicted in a stochastic acceleration scenario. A simplified temporal evolution model was developed and fitted to the SEDs and the resulting trends and phenomenology were characterised in the context of the theoretical literature. An expanding emission region model was also tested. Results. We find the spectral variability to be in good agreement with stochastic acceleration. Our analysis suggests that the standard LPPL distribution develops a Maxwellian pile-up component at the transition from the acceleration-dominated to the cooling-dominated phase on 3 nights in the dataset, as is also hinted at by the very-high energy and X-ray light curves. The resulting phenomenology of our sequential snapshot evolution SED model agrees well with theoretical and numerical simulation studies on temporal evolution using the diffusion equation approach. Curvature in the electron energy distribution (EED) anti-correlates with the synchrotron peak frequency, as was expected from stochastic acceleration. We tested an alternate model with expanding emission region and the resulting EED spectral index, expansion velocity, and magnetic field index agree with prior works using datasets collected in different flaring states of the source.

Very-high-energy gamma-ray intranight variability from BL Lacertae during the extreme flaring state of 2022

Astronomy and Astrophysics · 2026

Context. BL Lacertae (BL Lac), the archetypal blazar of its subclass and one of the most studied blazars of recent decades, has undergone a series of major multi-wavelength outbursts since 2020, resulting in its highest recorded γ-ray flare to date between September and November 2022, together with those from August 2021 and October 2024. Aims. We characterised the γ-ray and multi-wavelength emission and spectral energy distribution (SED) of BL Lac, as well as their evolution during the major and extended γ-ray and multi-wavelength flare that occurred between September and November 2022. Methods. We evaluated the variability of the flare, focusing on the nights of October 20 and November 13, 2022 when clear intranight very-high-energy (VHE, E &gt; 100 GeV) γ-ray variability was observed. We modelled the γ-ray and broadband SEDs during periods of stable emission identified with a Bayesian block analysis and interpreted the flare's evolution in terms of the variability in the relativistic particles and the jet's physical parameters. Results. During this flare, the VHE emission shows an average flux of 0.23 Crab units (C.U.) above 200 GeV and a variability amplitude of more than a factor of ten. We observe intranight flux-doubling variations as fast as ∼8 minutes during the nights of October 20 and November 13, 2022 with maximum fluxes of 4.4 C.U. above 100 GeV and 2.8 C.U. above 200 GeV. The spectral analysis reveals a transition of the X-ray emission from the high- to the low-energy SED peak and a shift in the γ-ray peak towards higher energies. We interpret the broadband emission within a leptonic two-zone model in which intranight variability is explained as magnetic reconnection in a compact region closely orientated with the line of sight, while variations in the relativistic electron distributions and the injection of freshly accelerated particles explain the weekly scale variations.

Extreme Blazars Observed with MAGIC: Second Catalog Release

The Astrophysical Journal · 2026

Extremely high-peaked BL Lac objects—also named extreme blazars—are among the most energetic and persistent extragalactic accelerators in the Universe, defined by a synchrotron emission peaking above 10<SUP>17</SUP> Hz in X-rays. Such emission is then reprocessed and produces radiation extending deeply into very-high-energy (VHE; energy E &gt; 100 GeV) gamma-rays. Observations in this energy band—optimally investigated by the Imaging Air-Shower Cherenkov telescopes—are crucial for probing the physical processes that drive their extreme behavior. This study extends our investigation of extreme blazars in the VHE gamma-ray range, providing a second new mini-catalog of sources observed by the MAGIC telescopes. We report on the monitoring of seven targets between 2017 and 2025, including four newly observed sources and three that have been part of long-term observation campaigns, for a total of approximately 338 hr of observations. The analysis of MAGIC data reveals two new VHE detections of extreme blazars, along with three additional sources showing hints of VHE emission. Joint observations of MAGIC and the first Large-Sized Telescope (LST-1) also confirmed a new VHE extreme blazar. Our results are complemented by simultaneous multiwavelength observations in other energy bands, including optical-UV, X-rays, and high-energy gamma-rays (100 MeV &lt; E &lt; 100 GeV). We confirm typical behavior of extreme blazars, such as a modest variability and a "harder-when-brighter" trend in X-rays across the sample. This new set increases the population of extreme blazars, and their broadband analysis confirms the physical properties of these extreme sources.

The IXPE and multifrequency polarimetric view of the extreme blazars 1ES 1101-232 and RGB J0710+591

Astronomy and Astrophysics · 2026

Multiwavelength polarimetry offers a powerful tool to probe magnetic field and flow geometries in the relativistic jets of blazars. Sources with synchrotron emission that spans a broad frequency range, from radio to X-rays, such as High Synchrotron Peak (HSP) type BL Lac objects, are particularly interesting. Previous measurements including radio, optical, and X-ray data show a clear trend, with the degree of polarization increasing with frequency. Here we report radio, optical, and X-ray observations (Swift, Nustar, and IXPE) of 1ES 1101-232 and RGB J0710+591, two blazars belonging to the puzzling subclass of extreme BL Lacs (EHBL). For 1ES 1101-232, we find a strong frequency dependence of the degree of polarization, with a ratio Π<SUB>X</SUB>/Π<SUB>O</SUB> ≃ 5.2. For RGB J0710+591, IXPE derives a 1σ upper limit Π<SUB>X</SUB> &lt; 11.6%, comparable to the measured optical degree of polarization (average Π<SUB>O</SUB> ∼ 12%). We discuss the results in the framework of current interpretations and, in particular, we report an improved version of the stratified shock model that reproduces the observed data of both sources.

SAETASS: Solver for Astroparticle Equation of Transport Analysis in Spherical Symmetry

Zenodo · 2026

A scientific computing package for solving astroparticle transport equation using operator splitting method and a finite volume approach.

Modular wavelength-shifting photon detector with SiPM readout for large-area applications

Journal of Instrumentation · 2026

Wavelength shifters (WLS) offer a scalable and affordable approach to large-area photon detection. They absorb photons and re-emit them at longer wavelengths, enabling efficient light trapping by total internal reflection. We present a compact detector module based on WLS tiles coupled to silicon photomultipliers (SiPMs). The design exploits the geometry of elongated photodetectors placed along the tile edges to maximize photon collection without requiring large sensor surfaces. Particular attention was given to matching the spectral response of the WLS and the SiPMs to the Cherenkov emission peak. Laboratory tests with a pulsed UV laser provide preliminary measurements of the photon-detection efficiency and pulse shape. For the single-shift configuration, a preliminary photon-detection efficiency of about 20% was measured. First pulse-shape studies indicate a signal width of approximately 5 ns FWHM, reflecting the impact of wavelength shifting and optical reflections on signal broadening. While the absolute efficiency is lower than conventional solutions, the modularity and scalability of the system make it a promising candidate for large-area photon detection. Further studies will address detailed timing resolution and array-level performance.

POLAMI Multiwavelength Polarization Study of Active Galactic Nuclei Jets: A Millimeter─Optical Comparison

The Astrophysical Journal Supplement Series · 2026

Millimeter-band polarimetry offers a powerful probe of active galactic nuclei jets, accessing regions less affected by opacity and Faraday rotation than at longer radio wavelengths. As part of the POLAMI program, we have conducted 14 yr of 1 and 3 mm polarization monitoring of a sample of gamma-ray-bright blazars with the IRAM 30 m telescope, complemented here with long-term optical polarimetric observations from multiple facilities. We aim to test whether current models of parsec-scale jet physics are consistent with observed multiband polarization behavior. Using a Bayesian framework, we derive intrinsic mean flux densities and modulation indices for total flux and fractional polarization, and characterize electric vector position angle (EVPA) variability using circular statistics. We then examine how these quantities reflecting variability properties across millimeter and optical bands relate to synchrotron peak frequency, jet orientation, and radio/gamma-ray luminosities. BL Lacertae (BL Lac) objects exhibit, on average, higher fractional polarization and lower EVPA variability than flat-spectrum radio quasars (FSRQs) at all wavelengths. Fractional polarization increases with frequency, consistent with increasingly ordered magnetic fields at shorter wavelengths. BL Lacs also show more frequent alignment of EVPAs between optical and millimeter bands, whereas FSRQs display weaker coherence. EVPA variability correlates positively with radio and gamma-ray luminosities and negatively with synchrotron peak frequency, most strongly in the optical. We further find a positive correlation between EVPA spread and fractional polarization variability, suggesting a direct link between magnetic-field structure and polarization dynamics.

Polarisation angle variability in tidal disruption events

Astronomy and Astrophysics · 2026

Context. Tidal disruption events (TDEs) occur when a star is disrupted by the tidal forces of a supermassive black hole, and these events produce bright multi-wavelength flares. Polarimetric measurements of TDEs allow us to disentangle the geometry and the mechanisms characterising the accretion process. Aims. We carried out the first systematic study of the time evolution of the optical polarisation angle (Θ) in a sample of classified TDEs, combining our own data with all available measurements from the literature, with the goal of testing the currently available models that describe TDE emission. Methods. We assembled data from all available observing epochs with significant linear polarisation detections (Π − 3σ<SUB>Π</SUB> &gt; 0%) for sources with at least two such epochs, and we determined the overall variability trends across the sample in various time frames, such as days from peak time and the fallback time (t<SUB>0</SUB>) derived from the different models. Results. Our final sample comprises 12 transients, including three Bowen fluorescence flares (BFFs). The majority of the sources show significant Θ variability. The distribution of |dΘ/dt| peaks near ∼2° d<SUP>−1</SUP>. BFFs tend to display sustained late-time Θ evolution, likely due in part to their slower fading. No universal trend emerges when time is normalised by t<SUB>0</SUB>. Conclusions. Short-timescale Θ variability is common in TDEs and is difficult to reconcile with simple axisymmetric reprocessing models that predict a constant polarisation angle. The observed phenomenology favours scenarios with evolving, non-axisymmetric geometries and/or shocks, possibly coupled with changes in optical depth. Denser polarimetric monitoring, contemporaneous spectroscopy, and X-ray/UV coverage are required to break the remaining degeneracies.

Combined dark matter search towards dwarf spheroidal galaxies with Fermi-LAT, HAWC, H.E.S.S., MAGIC, and VERITAS

Journal of Cosmology and Astroparticle Physics · 2026

Dwarf spheroidal galaxies (dSphs) are excellent targets for indirect dark matter (DM) searches using gamma-ray telescopes because they are thought to have high DM content and a low astrophysical background. The sensitivity of these searches is improved by combining the observations of dSphs made by different gamma-ray telescopes. We present the results of a combined search by the most sensitive currently operating gamma-ray telescopes, namely: the satellite-borne Fermi-LAT telescope; the ground-based imaging atmospheric Cherenkov telescope arrays H.E.S.S., MAGIC, and VERITAS; and the HAWC water Cherenkov detector. Individual datasets were analyzed using a common statistical approach. Results were subsequently combined via a global joint likelihood analysis. We obtain constraints on the velocity-weighted cross section &lt;σv&gt; for DM self-annihilation as a function of the DM particle mass. This five-instrument combination allows the derivation of up to 2-3 times more constraining upper limits on &lt;σv&gt; than the individual results over a wide mass range spanning from 5 GeV to 100 TeV. Depending on the DM content modeling, the 95% confidence level observed limits reach 1.5×10<SUP>-24</SUP> cm<SUP>3</SUP>s<SUP>-1</SUP> and 3.2×10<SUP>-25</SUP> cm<SUP>3</SUP>s<SUP>-1</SUP>, respectively, in the τ <SUP>+</SUP> τ <SUP>-</SUP> annihilation channel for a DM mass of 2 TeV.

First detection of VHE gamma-ray signal from the FSRQ TON 0599

Monthly Notices of the Royal Astronomical Society · 2026

TON 0599 (z = 0.7247) belongs to the few flat spectrum radio quasars (FSRQs) detected in the very high energy (VHE, <inline-formula><tex-math>$E &gt; 100$</tex-math></inline-formula> GeV) gamma-ray band. Its redshift makes it currently one of the farthest VHE gamma-ray sources. It was detected for the first time with the MAGIC telescopes on 2017 December 15, and observed until December 29. The flux reached a maximum of about 50 per cent of the Crab Nebula flux above 80 GeV on the second night of observation, after which we witnessed a gradual decrease of the flux. The VHE gamma-ray spectrum connects smoothly to the one in the high energy (<inline-formula><tex-math>$E &gt; 100$</tex-math></inline-formula> MeV) band obtained from simultaneous observations with Fermi-LAT. It features a cut-off at energies around 50 GeV, indicating the location of the gamma-ray emission zone beyond the broad line region. In addition, we were able to follow the spectral evolution during the fading phase of the flare. Multiwavelength analysis based on observations in optical, near-infrared, and radio bands acquired by the Whole Earth Blazar Telescope Collaboration from November to March, as well as observations in X-ray and optical─UV bands with instruments on board the Swift satellite, shows strong correlation between different bands. We model the broadband emission with a simple one-zone leptonic model, where the high-energy peak is predominantly produced by external Compton scattering of photons from the dusty torus.

A long-term multiwavelength study of the flat spectrum radio quasar OP 313

Astronomy and Astrophysics · 2026

Context. The flat spectrum radio quasar OP 313, is a high-redshift (z = 0.997) blazar that entered an intense γ-ray active phase from November 2023 to March 2024, as observed by the as observed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. Aims. We present a multiwavelength analysis covering 15 years of data, from August 2008 to March 2024, to contextualize this period of extreme γ-ray activity within the long-term emission of the source. Methods. We analyzed a long-term, comprehensive, multiwavelength dataset from different facilities and projects from radio to γ rays. We identified the seven most intense γ-ray flaring periods and performed a kinematic analysis of Very Long Baseline Array (VLBA) data to determine whether new jet components emerged before or during these flares. For two of these flaring periods, we performed the modeling of the spectral energy distribution (SED). Results. The VLBA-BU-BLAZAR and MOJAVE datasets reveal a new jet component appearing in both visibility datasets prior to the onset of one of the strongest γ-ray flares. By comparing the timing of the VLBA-BU-BLAZAR knots' ejection with the γ-ray flaring periods, we constrained the setup of the SED modeling. We also found that the first γ-ray flaring period is less Compton-dominated than the others. Conclusions. Our results suggest that the recent activity of OP 313, is triggered by new jet components emerging from the core and interacting with a standing shock. The γ-ray emission likely arises from dusty torus photons upscattered via inverse Compton (IC) by relativistic jet electrons. The SED modeling indicates that this component is less dominant during the first γ-ray flaring period than the later ones.

Time-dependent Modeling of the Subhour Spectral Evolution during the 2013 Outburst of Mrk 421

The Astrophysical Journal · 2026

In 2013 April, the TeV blazar Markarian 421 underwent one of its most powerful emission outbursts recorded to date. An extensive multi-instrument campaign featuring MAGIC, VERITAS, and NuSTAR provided comprehensive very high-energy (VHE; E &gt; 100 GeV) and X-ray coverage over nine consecutive days. The VHE flux peaked at approximately 15 times that of the Crab Nebula, with rapid variability detected on timescales down to 15 minutes in both X-ray and VHE bands. This rich data set, characterized by its dense temporal coverage and high photon statistics, offers an unparalleled opportunity to probe the broadband emission dynamics in blazars. In this work, we perform a detailed spectral analysis of the X-ray and VHE emissions on subhour timescales throughout the flare. We identify several clockwise spectral hysteresis loops in the X-rays, revealing a spectral evolution more complex than a simple harder-when-brighter trend. The VHE spectrum extends beyond 10 TeV, and its temporal evolution closely mirrors the behavior in the X-rays. Crucially, we report the first evidence of VHE spectral hysteresis occurring simultaneously with the X-ray loops. To interpret these findings, we apply a time-dependent leptonic model to 240 broadband spectral energy distributions (SEDs) binned on a 15 minute scale, allowing us to self-consistently track the particle distribution's history. Our modeling shows that the majority of the subhour flux and spectral variations are driven by changes in the luminosity and slope of the injected electron distribution. The required variations in the electron slope are difficult to reconcile with magnetic reconnection, but they are consistent with a shock-acceleration scenario where the shock compression ratio evolves by a factor of ∼2. The model also points to a relatively stable magnetic field and emitting region size, favoring a scenario where the emission originates from a stationary feature in the jet, such as a recollimation shock. However, this scenario requires a jet Lorentz factor that significantly exceeds values from VLBI measurements to account for the high minimum electron energy implied by the lack of variability in the optical band.

Prompt Searches for Very-high-energy γ-Ray Counterparts to IceCube Astrophysical Neutrino Alerts

The Astrophysical Journal · 2026

The search for sources of high-energy astrophysical neutrinos can be significantly advanced through a multimessenger approach, which seeks to detect the γ-rays that accompany neutrinos as they are produced at their sources. Multimessenger observations have so far provided the first evidence for a neutrino source, illustrated by the joint detection of the flaring blazar TXS 0506+056 in high-energy (E &gt; 1 GeV) and very-high-energy (VHE; E &gt; 100 GeV) γ-rays in coincidence with the high-energy neutrino IceCube-170922A, identified by IceCube. Imaging atmospheric Cherenkov telescopes (IACTs), namely FACT, H.E.S.S., MAGIC, and VERITAS, continue to conduct extensive neutrino target-of-opportunity follow-up programs. These programs have two components: follow-up observations of single astrophysical neutrino candidate events (such as IceCube-170922A), and observation of known γ-ray sources after the identification of a cluster of neutrino events by IceCube. Here we present a comprehensive analysis of follow-up observations of high-energy neutrino events observed by the four IACTs between 2017 September (after the IceCube-170922A event) and 2021 January. Our study found no associations between γ-ray sources and the observed neutrino events. We provide a detailed overview of each neutrino event and its potential counterparts. Furthermore, a joint analysis of all IACT data is included, yielding combined upper limits on the VHE γ-ray flux.

CTAO LST─1 observations of magnetar SGR 1935+2154: Deep limits on sub-second bursts and persistent tera-electronvolt emission

Astronomy and Astrophysics · 2026

Context. The Galactic magnetar SGR 1935+2154 has exhibited prolific high-energy (HE) bursting activity in recent years. Aims. Investigating its potential tera-electronvolt counterpart could provide insights into the underlying mechanisms of magnetar emission and very high-energy (VHE) processes in extreme astrophysical environments. We aim to search for a possible tera-electronvolt counterpart to both its persistent and sub-second-scale burst emission. Methods. We analysed over 25 hour of observations from the Large-Sized Telescope prototype (LST−1) of the Cherenkov Telescope Array Observatory (CTAO) during periods of HE activity from SGR 1935+2154 in 2021 and 2022 to search for persistent emission. For bursting emission, we selected and analysed nine 0.1 s time windows centred around known short X-ray bursts, targeting potential sub-second-scale tera-electronvolt counterparts in a low-photon-statistics regime. Results. While no persistent or bursting emission was detected in our search, we establish upper limits for the tera-electronvolt emission of a short magnetar burst simultaneous to its soft gamma-ray flux. Specifically, for the brightest burst in our sample, the ratio between tera-electronvolt and X-ray flux is ≲10<SUP>−3</SUP>. Conclusions. The non-detection of either persistent or bursting tera-electronvolt emission from SGR 1935+2154 suggests that if such components exist, they may occur under specific conditions not covered by our observations. This aligns with theoretical predictions of VHE components in magnetar-powered fast radio bursts and the detection of MeV─GeV emission in giant magnetar flares. These findings underscore the potential of magnetars, fast radio bursts, and other fast transients as promising candidates for future observations in the low-photon-statistics regime with Imaging Atmospheric Cherenkov Telescopes, particularly with the CTAO.

Intra-night optical polarization monitoring of blazars

Astronomy and Astrophysics · 2025

Blazars are known for their extreme variability across the electromagnetic spectrum. Variability at very short timescales can allow us to discriminate between competing models. This is particularly true for polarization variability, which allows us to probe particle acceleration and high-energy emission models in blazars. Here we present results from the first pilot study of intra-night optical polarization monitoring conducted using RoboPol at the Skinakas Observatory; these results are supplemented by observations from the Calar Alto, Perkins, and Sierra Nevada observatories. Our results show that while variability patterns can vary widely between sources, variability on timescales as short as minutes is prevalent in blazar jets. The amplitudes of the variations are typically small, a few percent for the polarization degree and less than 20° for the polarization angle, pointing to a significant contribution to the optical emission from a turbulent magnetic field component. The overall stability of the polarization angle over time points to a preferred magnetic field orientation.

Monitoring of 3C 286 with ALMA, IRAM, and SMA from 2006 to 2025: Stability, Synchrotron Ages, and Frequency-dependent Polarization Attributed to Core-shift

The Astrophysical Journal · 2025

We present the results of multifrequency monitoring of the radio quasar 3C 286, conducted using three instruments: the Atacama Large Millimeter/submillimeter Array (ALMA) at 91.5, 103.5, 233.0, and 343.4 GHz, the IRAM 30 m Telescope at 86 and 229 GHz, and the Submillimeter Array at 225 GHz. The IRAM measurements from 2006 to 2024 show that the total flux of 3C 286 is stable within measurement uncertainties, indicating long-term stability up to 229 GHz, when applying a fixed kelvin-to-jansky conversion factor throughout its data set. ALMA data from 2018 to 2024 exhibit a decrease in flux, of which up to 4% could be attributed to an apparent increase in the absolute brightness of Uranus, the primary flux calibrator for ALMA with the ESA4 model. Taken together, these results suggest that the intrinsic total flux of 3C 286 has remained stable up to 229 GHz over the monitoring period. The polarization properties of 3C 286 are stable across all observing frequencies. The electric vector position angle (EVPA) gradually rotates as a function of wavelength squared, which is well described by a single power law over the full frequency range. We therefore propose using the theoretical EVPA values from this model curve for absolute EVPA calibration between 5 and 343.4 GHz. The Faraday rotation measure (RM) increases as a function of frequency up to (3.2 ± 1.5) × 10<SUP>4</SUP> rad m<SUP>−2</SUP>, following ∣RM∣ ∝ ν<SUP>α</SUP> with α = 2.05 ± 0.06. This trend is consistent with the core-shift effect expected in a conical jet.

Multiband optical variability on diverse timescales of the blazar Ton 599 from 2011 to 2023

Astronomy and Astrophysics · 2025

Context. We analyze the optical variability of the flat-spectrum radio quasar (FSRQ) Ton 599 using BVRI photometry from the Whole Earth Blazar Telescope (WEBT) collaboration (2011─2023), complemented by photometric and spectroscopic data from the Steward Observatory monitoring program. Aims. We aim to characterize short- and long-term optical variability ─ including flux distributions, intranight changes, color evolution, and spectra ─ to constrain physical parameters and processes in the central engine of this active galactic nucleus (AGN). Methods. We tested flux distributions in each filter against normal and log-normal models and explored the root mean square (RMS)─flux relation. We derived power spectral densities (PSDs) to assess red-noise behavior. We quantified intranight variability using a χ<SUP>2</SUP> test and fractional variability. From variability timescales, we estimated the emitting region size and magnetic field. Long-term variability was studied by segmenting the light curve into 12 intervals and analyzing flux statistics. For multi-filter flares, we computed spectral slopes, redshift-corrected fluxes, and monochromatic luminosities. Color-magnitude and color-time diagrams traced color evolution over different flux regimes and timescales. From low-flux spectra, we measured Mg II line properties (correcting for Fe II) to estimate the black hole mass via single-epoch scaling. Results. During the monitoring period, Ton 599 showed strong optical variability. Log-normal distributions fit the fluxes better than normal ones, and all bands display a positive RMS─flux relation. The PSDs follow red-noise trends. Intranight variability is detected, with derived timescales constraining the emission region and magnetic field. The R band reaches a peak flux of 23.5 mJy, corresponding to a monochromatic luminosity of log(νL<SUB>ν</SUB>) = 48.48 [erg s<SUP>−1</SUP>]. Color-magnitude diagrams reveal a redder-when-brighter trend at low fluxes (thermal dominance), achromatic behavior at intermediate levels (possibly due to jet orientation changes), and a bluer-when-brighter trend at high fluxes (synchrotron dominance). While long-term color changes are modest, short-term variations are significant, with a negative correlation between the amplitude of color changes and the average flux. The estimated supermassive black hole mass is on the order of 10<SUP>8</SUP> M<SUB>⊙</SUB>, which is in agreement with previous estimates. Conclusions. Our results underscore the complexity of blazar variability, pointing to multiple emission processes at work. The joint photometric and spectroscopic approach constrains key physical parameters and deepens our understanding of the blazar central engine.

Prospects for dark matter observations in dwarf spheroidal galaxies with the Cherenkov Telescope Array Observatory

Monthly Notices of the Royal Astronomical Society · 2025

The dwarf spheroidal galaxies (dSphs) orbiting the Milky Way are widely regarded as systems supported by velocity dispersion against self-gravity, and as prime targets for the search for indirect dark matter (DM) signatures in the GeV-to-TeV <inline-formula><tex-math>$\gamma$</tex-math></inline-formula>-ray range owing to their lack of astrophysical <inline-formula><tex-math>$\gamma$</tex-math></inline-formula>-ray background. We present forecasts of the sensitivity of the forthcoming Cherenkov Telescope Array Observatory (CTAO) to annihilating or decaying DM signals in these targets. An original selection of candidates is performed from the current catalogue of known objects, including both classical and ultrafaint dSphs. For each, the expected DM content is derived using the most comprehensive photometric and spectroscopic data available, within a consistent framework of analysis. This approach enables the derivation of novel astrophysical factor profiles for indirect DM searches, which are compared with results from the literature. From an initial sample of 64 dSphs, eight promising targets are identified ─ Draco I, Coma Berenices, Ursa Major II, Ursa Minor, and Willman 1 in the North, Reticulum II, Sculptor, and Sagittarius II in the South ─ for which different DM density models yield consistent expectations, leading to robust predictions. CTAO is expected to provide the strongest limits above <inline-formula><tex-math>$\sim$</tex-math></inline-formula>10 TeV, reaching velocity-averaged annihilation cross sections of <inline-formula><tex-math>$\sim 5\times 10^{-25}$</tex-math></inline-formula> cm<inline-formula><tex-math>$^3$</tex-math></inline-formula> s<inline-formula><tex-math>$^{-1}$</tex-math></inline-formula> and decay lifetimes up to <inline-formula><tex-math>$\sim 10^{26}$</tex-math></inline-formula> s for combined limits. The dominant uncertainties arise from the imprecise determination of the DM content, particularly for ultrafaint dSphs. Observation strategies are proposed that optimize either deep exposures of the best candidates or diversified target selections.

Unveiling blazar synchrotron emission: A multiwavelength polarimetric study of high-synchrotron and low-synchrotron peaked populations

Astronomy and Astrophysics · 2025

The polarimetric properties of blazars enable us to place constraints on the acceleration mechanisms that fuel their powerful jets. By studying the multiwavelength polarimetric behaviour of high-synchrotron peaked (HSP) and low-synchrotron peaked (LSP) blazars, we aim to explore differences in their emission mechanisms and magnetic field structure in the acceleration region. In this study, we take advantage of several X-ray polarisation observations of HSP by the IXPE, including four new observations of Mrk 501, along with optical polarisation observations of LSP from RoboPol and other instruments. We find that the polarisation degree (PD) distribution of HSP in X-rays is systematically higher than in optical and mm-radio wavelengths, as reported in previous IXPE publications. The distribution of the X-ray electric vector position angles (PA) is centred around the jet axis with most of the observations consistent with zero difference within uncertainties. In fact, the distribution of the offset of the PA from the jet axis is consistent between the LSP and HSP populations (with PA measured in optical for the first, X-ray for the latter), suggesting a common magnetic field structure close to the acceleration region. These results offer strong support for the emerging energy stratified scenario of particle acceleration followed by energy loss in blazar jets.

Detection of Compton Scattering in the Jet of 3C 84

The Astrophysical Journal · 2025

3C 84 is the brightest cluster galaxy in the Perseus Cluster. It is among the closest radio-loud active galaxies and among the very few that can be detected from low-frequency radio up to TeV γ-rays. Here we report on the first X-ray polarization observation of 3C 84 with the Imaging X-ray Polarimetry Explorer, for a total of 2.2 Ms coinciding with a flare in γ-rays. This is the longest observation for a radio-loud active galaxy, which allowed us to reach unprecedented sensitivity, leading to the detection of an X-ray polarization degree of Π<SUB>X</SUB> = 4.2% ± 1.3% (∼3.2σ confidence) at an X-ray electric vector polarization angle of ψ<SUB>X</SUB> = 163° ± 9°, which is aligned with the radio jet direction on the sky. Optical polarization observations show fast variability about the jet axis as well. Our results strongly favor models in which X-rays are produced by Compton scattering from relativistic electrons—specifically synchrotron self-Compton—that takes place downstream, away from the supermassive black hole.

Polarimetric diversity in tidal disruption events: Comparative study of low-polarised sources with AT2020mot

Astronomy and Astrophysics · 2025

Context. Tidal disruption events (TDEs) occur when a star is disrupted by the tidal forces of a supermassive black hole (SMBH), which produces bright multi-wavelength flares. Among these events, AT2020mot has so far exhibited the highest recorded optical polarisation, with tidal shocks proposed as the primary source of its polarised emission. Aims. We present a comprehensive analysis of 13 TDEs with available polarimetric observations, aiming to determine whether the unusually high polarisation of AT2020mot stems from unique physical processes or arises from mechanisms shared by other TDEs. Methods. We present new optical polarisation measurements of TDEs obtained from multiple ground-based telescopes, combining them with optical, UV, and X-ray light curves from the Zwicky Transient Facility and the Swift observatory. We derived intrinsic TDE properties ─ such as SMBH and stellar masses ─ using MOSFiT and TDEMass, and compared them with those of the sample population. Results. Our population study reveals that AT2020mot aligns with the broader TDE sample in terms of most physical properties, including blackbody temperature, luminosity, and rise timescales. However, its optical polarisation degree is exceptionally high compared to the low or undetected polarisation observed in other events. Additionally, according to our MOSFiT fit, AT2020mot has an elevated column density, which suggests a more complex environment than is typically assumed. Conclusions. We conclude that although AT2020mot fits well within the general TDE population in terms of global characteristics, its extraordinarily high polarisation and higher column density challenge current models based purely on shock or reprocessing mechanisms. More extensive, time-resolved polarimetric monitoring of newly discovered TDEs will be critical to determine whether AT2020mot represents an outlier or the extreme end of a continuum of TDE properties.

VHE γ-ray observations of bright BL Lacs with the Large-Sized Telescope prototype (LST-1) of the CTAO

Monthly Notices of the Royal Astronomical Society · 2025

Cherenkov Telescope Array Observatory (CTAO) is the next-generation ground-based <inline-formula><tex-math>$\gamma$</tex-math></inline-formula>-ray observatory operating in the energy range from <inline-formula><tex-math>$20\, \mathrm{GeV}$</tex-math></inline-formula> up to <inline-formula><tex-math>$300\, \mathrm{TeV}$</tex-math></inline-formula>, with two sites in La Palma (Spain) and Paranal (Chile). It will consist of telescopes of three sizes, covering different parts of the large energy range. We report on the performance of Large-Sized Telescope prototype (LST-1) in the detection and characterization of extragalactic <inline-formula><tex-math>$\gamma$</tex-math></inline-formula>-ray sources, with a focus on the reconstructed <inline-formula><tex-math>$\gamma$</tex-math></inline-formula>-ray spectra and variability of classical bright BL Lacertae objects, which were observed during the early commissioning phase of the instrument. LST-1 data from known bright <inline-formula><tex-math>$\gamma$</tex-math></inline-formula>-ray blazars ─ Markarian 421, Markarian 501, 1ES 1959+650, 1ES 0647+250, and PG 1553 + 113 ─ were collected between 2020 July 10, and 2022 May 23, covering a zenith angle range of 4<inline-formula><tex-math>$^\circ$</tex-math></inline-formula> to 57<inline-formula><tex-math>$^\circ$</tex-math></inline-formula>. The reconstructed light curves were analysed using a Bayesian block algorithm to distinguish the different activity phases of each blazar. Simultaneous Fermi-LAT data were utilized to reconstruct the broad-band <inline-formula><tex-math>$\gamma$</tex-math></inline-formula>-ray spectra for the sources during each activity phase. High-level reconstructed data in a format compatible with gammapy are provided together with measured light curves and spectral energy distributions (SEDs) for several bright blazars and an interpretation of the observed variability in long and short time-scales. Simulations of historical flares are generated to evaluate the sensitivity of LST-1. This work represents the first milestone in monitoring bright BL Lacertae objects with a CTAO telescope.

Very high energy observations of the Seyfert galaxy NGC 4151 with MAGIC: Indication of another gamma-ray obscured candidate neutrino source

Astronomy and Astrophysics · 2025

Seyfert galaxies are emerging as a promising source class of high-energy neutrinos. The Seyfert galaxies NGC 4151 and NGC 1068 have respectively come up as the most promising counterparts of a 3σ and of a 4.2σ neutrino excesses detected by IceCube in the TeV energy range. Constraining the very high energy (VHE) emission associated with the neutrino signal is crucial to unveiling the mechanism and site of neutrino production. In this work, we present the first results of the VHE observations (∼29 hours) of NGC 4151 with the MAGIC telescopes. We detected no gamma-ray excess in the direction of NGC 4151, and we derived constraining upper limits on the VHE gamma-ray flux. The integral flux upper limit (at the 95% confidence level) above 200 GeV is f = 2.3 × 10<SUP>−12</SUP> cm<SUP>−2</SUP> s<SUP>−1</SUP>. Comparison of the MAGIC and IceCube measurements suggests the presence of a gamma-ray obscured accelerator, and it allowed us to constrain the gamma-ray optical depth and the size of the neutrino production site.

Constraining the TeV gamma-ray emission of SN 2024bch, a possible type IIn-L from a red supergiant progenitor: Multiwavelength observations and analysis of the progenitor

Astronomy and Astrophysics · 2025

We present very high-energy optical photometry and spectroscopic observations of SN 2024bch in the nearby galaxy NGC 3206 (∼20 Mpc). We used gamma-ray observations performed with the first Large-Sized Telescope (LST-1) of the Cherenkov Telescope Array Observatory (CTAO) and optical observations with the Liverpool Telescope (LT) combined with data from public repositories to evaluate the general properties of the event and the progenitor star. No significant emission above the LST-1 energy threshold for this observation (∼100 GeV) was detected in the direction of SN 2024bch, and we computed an integral upper limit on the photon flux of F<SUB>γ</SUB>(&gt; 100 GeV)≤3.61 × 10<SUP>−12</SUP> cm<SUP>−2</SUP> s<SUP>−1</SUP> based on six nonconsecutive nights of observations with the LST-1, between 16 and 38 days after the explosion. Employing a general model for the gamma-ray flux emission, we found an upper limit on the mass-loss-rate to wind-velocity ratio of <inline-formula> Ṁ/u<SUB>w</SUB> ≤ 10<SUP>−4</SUP> M<SUB>⊙</SUB>/ yr s/km <mml:math> <mml:mrow> <mml:mover> <mml:mi>M</mml:mi> <mml:mo>˙</mml:mo> </mml:mover> <mml:mo>/</mml:mo> <mml:msub> <mml:mi>u</mml:mi> <mml:mrow> <mml:mtext>w</mml:mtext> </mml:mrow> </mml:msub> <mml:mo>≤</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> <mml:mfrac> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mrow> <mml:mspace></mml:mspace> <mml:mtext>yr</mml:mtext> </mml:mrow> </mml:mfrac> <mml:mfrac> <mml:mrow> <mml:mtext>s</mml:mtext> </mml:mrow> <mml:mrow> <mml:mspace></mml:mspace> <mml:mtext>km</mml:mtext> </mml:mrow> </mml:mfrac> </mml:mrow> </mml:math> </inline-formula>, although gamma-gamma absorption could potentially have skewed this estimation, effectively weakening our constraint. From spectro-photometric observations we found progenitor parameters of M<SUB>pr</SUB> = 11 ─ 20 M<SUB>⊙</SUB> and R<SUB>pr</SUB> = 531 ± 125 R<SUB>⊙</SUB>. Finally, using archival images from the Hubble Space Telescope, we constrained the luminosity of the progenitor star to log (L<SUB>pr</SUB>/L<SUB>⊙</SUB>) ≤ 4.82 and its effective temperature to T<SUB>pr</SUB> ≤ 4000 K. Our results suggest that SN 2024bch is a type IIn-L supernova that originated from a progenitor star consistent with a red supergiant. We show how the correct estimation of the mass-loss history of a supernova will play a major role in future multiwavelength observations.

The Most Distant γ-Ray Flare to Date: A Multiwavelength Campaign on the z = 4.715 Blazar GB6 B1428+4217

The Astrophysical Journal · 2025

In 2023 November, the Fermi Large Area Telescope detected a γ-ray flare from the high-redshift blazar GB6 B1428+4217 (z = 4.715). We initiated a multiwavelength follow-up campaign involving Swift, NuSTAR, the Sierra Nevada and Perkins Observatories, and the Effelsberg 100 m radio telescope. This source, also known as 5BZQ J1430+4204, has shown an anomalous soft X-ray spectrum in previous observations, including possible ionized absorption features or signatures of bulk Comptonization of thermal electrons, which are also detected during the flaring episode. Simultaneous optical data revealed a polarization fraction of ∼8% in the R band, confirming that synchrotron emission dominated over thermal emission from the accretion disk. The hard X-ray flux was enhanced during the flare. Modeling of the broadband spectral energy distribution suggests that the high-energy component is dominated by Compton scattering by external seed photons from the accretion disk. The origin of the flare is consistent with the injection of a hard-spectrum electron population in the emission region. With a γ-ray luminosity among the top 5% of flaring events, GB6 B1428+4217 exemplifies a prototypical MeV blazar. Its Compton-dominated spectral energy distribution and extreme luminosity are in line with expectations from the blazar sequence. High-redshift flares like this are critical for understanding jet physics in the early Universe and may improve detection prospects with future missions such as the Compton Spectrometer and Imager.

X-Ray Polarization Detection of the Pulsar Wind Nebula in G21.5─0.9 with IXPE

The Astrophysical Journal · 2025

We present the X-ray polarization observation of G21.5−0.9, a young Galactic supernova remnant (SNR), conducted with the Imaging X-ray Polarimetry Explorer (IXPE) in 2023 October, with a total livetime of approximately 837 ks. Using different analysis methods, such as a space-integrated study of the entire region of the pulsar wind nebula (PWN) and a space-resolved polarization map, we detect significant polarization from the PWN at the center of the SNR, with an average polarization degree of ∼10% oriented at ∼33° (north through east). No significant energy-dependent variation in polarization is observed across the IXPE band (2─8 keV). The polarization map, corrected for the effect of polarization leakage, reveals a consistent pattern in both degree and angle, with little change across the nebula. Our findings indicate the presence of a highly polarized central torus, suggesting low levels of turbulence at particle acceleration sites. Unlike Vela, but similar to the Crab Nebula, we observe substantial differences between radio and X-ray polarization maps. This suggests a clear separation in energy of the emitting particle populations and hints at an important, yet poorly understood, role of instabilities in the turbulence dynamics of PWNe.

Contemporaneous X-Ray and Optical Polarization of Extremely High-synchrotron-peaked Blazar H 1426+428

The Astrophysical Journal · 2025

We present the first contemporaneous X-ray and optical polarimetric measurement of the extremely high-synchrotron-peaked (HSP) blazar H 1426+428. The X-ray polarimetric observations were undertaken using the Imaging X-ray Polarimetry Explorer (IXPE) on 2024 May 27 and 2024 July 5. The IXPE pointings were accompanied by contemporaneous optical observations of the Observatorio de Sierra Nevada, Calar Alto Observatory, and the Perkins Telescope Observatory. While we observed the X-ray degree of polarization to be &gt;20%, the polarization in the optical band was found to be only 1%─3%. This trend has been observed in several HSP blazars with available optical and X-ray polarimetric data and is typically explained in terms of energy stratification downstream of a shock. However, we observed a significant difference between the optical and X-ray polarization angles, a feature that has been observed in certain HSP blazars, such as Mrk 421, but remains a relatively rare or underreported phenomenon. We discuss possible scenarios for these findings within the framework of a partially turbulent jet model.

Testing the ubiquitous presence of very high energy emission in gamma-ray bursts with the MAGIC telescopes

Astronomy and Astrophysics · 2025

Gamma-ray bursts (GRBs) are the most powerful transient objects in the Universe, and they are a primary target for the MAGIC Collaboration. Recognizing the challenges of observing these elusive objects with Imaging Atmospheric Cherenkov Telescopes (IACTs), we implemented a dedicated observational strategy that included an automated procedure for rapid re-pointing to transient sources. Since 2013, this automated procedure has enabled MAGIC to observe GRBs at a rate of approximately ten per year, which led to the successful detection of two GRBs at very high energies (VHE; E &gt; 100 GeV). We present a comprehensive analysis of 42 non-detected GRBs (4 short GRBs) observed by MAGIC from 2013 to 2019. We derived upper limits (ULs) on the observed energy flux as well as on the intrinsic energy flux corrected for absorption by the extragalactic background light (EBL) from the MAGIC observations in selected energy and time intervals. We conducted a comprehensive study of their properties to investigate the reasons for these non-detections, including the possible peculiar properties of TeV-detected GRBs. We find that strong EBL absorption significantly hinders TeV detection for the majority of GRBs in our sample. For a subset of 6 GRBs with redshift z &lt; 2, we compared the UL on the intrinsic flux in the VHE domain with the simultaneous X-ray flux, which is observed to be at the same level in the current population of TeV-detected GRBs. Based on these inferred MAGIC ULs, we conclude that a VHE component with a luminosity comparable to the simultaneously observed X-ray luminosity cannot be ruled out for this sample.

cta-observatory/cta-lstchain: v0.11.3

Zenodo · 2025

What's Changed pin ctapipe_io_lst 0.27.2 (#1378) @moralejo Fix ff dl1check (#1379) @moralejo Added command-line option to fix R0V files which lack pixel status info (#1377) @moralejo Fix pixel status in R0V files (#1376) @moralejo Contributers @moralejo

GRB 221009A: Observations with LST-1 of CTAO and Implications for Structured Jets in Long Gamma-Ray Bursts

The Astrophysical Journal · 2025

GRB 221009A is the brightest gamma-ray burst (GRB) observed to date. Extensive observations of its afterglow emission across the electromagnetic spectrum were performed, providing the first strong evidence of a jet with a nontrivial angular structure in a long GRB. We carried out an extensive observation campaign in very-high-energy (VHE) gamma rays with the first Large-Sized Telescope of the future Cherenkov Telescope Array Observatory starting on 2022 October 10, about 1 day after the burst. A dedicated analysis of the GRB 221009A data is performed to account for the different moonlight conditions under which data were recorded. We find an excess of gamma-like events with a statistical significance of 4.1σ during the observations taken 1.33 days after the burst, followed by background-compatible results for the later days. The results are compared with various models of afterglows from structured jets that are consistent with the published multiwavelength data but entail significant quantitative and qualitative differences in the VHE emission after 1 day. We disfavor models that imply VHE flux at 1 day considerably above 10<SUP>−11</SUP> erg cm<SUP>−2</SUP> s<SUP>−1</SUP>. Our late-time VHE observations can help disentangle the degeneracy among the models and provide valuable new insight into the structure of GRB jets.

Hunting star forming galaxies in the gamma-ray domain

Astronomy and Astrophysics · 2025

Context. Star-forming galaxies emit γ rays with relatively low luminosity, but the study of their emission is no less captivating. While it is known that their γ-ray luminosity in the GeV band is strongly linked to their star formation, the origin of their emission at higher energies remains uncertain due to limited observations. Aims. Our aim is to assemble the largest possible sample of star-forming galaxies with potential detectability by the new generation of Cherenkov telescopes. Methods. To achieve this, we compile a comprehensive sample of galaxies, including those previously detected by Fermi-LAT in the GeV energy range, as well as a larger sample of star-forming galaxies in the Local Volume that have been cataloged in the near-infrared band. We estimate their γ-ray flux by assuming a proportional relationship with their star formation rate, and then select the brightest candidates. The predicted spectra in the TeV band are derived using a simple empirical model normalized to the star formation rate and a model based on extrapolating the latest Fermi-LAT data to higher energies. The ground-based detectability of γ-ray emission from these sources is assessed through a comparison with the most recent instrument response functions. Results. Our investigation reveals that almost a dozen star-forming galaxies may be detectable by upcoming γ-ray telescopes. Conclusions. The observation of numerous star-forming galaxies in the TeV band is a fundamental piece of the panchromatic puzzle for understanding the physics inside these galaxies. The significant increase in the number of galaxies that will be able to be studied in detail in the near future, particularly with the Cherenkov Telescope Array Observatory, promises a major step forward in the study of the conditions of acceleration and transport of cosmic rays in nearby extragalactic environments.

Detection of the Geminga pulsar at energies down to 20 GeV with the LST-1 of CTAO

Astronomy and Astrophysics · 2025

Context. Geminga is the third gamma-ray pulsar firmly detected by imaging atmospheric Cherenkov telescopes (IACTs) after the Crab and the Vela pulsars. Most of its emission is expected at tens of giga-electronvolts, and, out of the planned telescopes of the upcoming Cherenkov Telescope Array Observatory (CTAO), the Large-Sized Telescopes (LSTs) are the only ones with optimised sensitivity at these energies. Aims. We aim to characterise the gamma-ray pulse shape and spectrum of Geminga as observed by the first LST (hereafter LST-1) of the Northern Array of CTAO. Furthermore, this study confirms the great performance and the improved energy threshold of the telescope, as low as 10 GeV for pulsar analysis, with respect to current-generation Cherenkov telescopes. Methods. We analysed 60 hours of good-quality data taken by the LST-1 between December 2022 and March 2024 at zenith angles below 50°. Additionally, a new Fermi-LAT analysis of 16.6 years of data was carried out to extend the spectral analysis down to 100 MeV. Lastly, a detailed study of the systematic effects was performed. Results. We report the detection of Geminga in the energy range between 20 and 65 GeV. Of the two peaks of the phaseogram, the second one, P2, is detected with a significance of 12.2σ, while the first (P1) reaches a significance level of 2.6σ. The best-fit model for the spectrum of P2 was found to be a power law with a spectral index of Γ = (4.5 ± 0.4<SUB>stat</SUB>)<SUB>−0.6<SUB>sys</SUB></SUB><SUP>+0.2<SUB>sys</SUB></SUP>, compatible with the previous results obtained by the MAGIC Collaboration. No evidence of curvature is found in the LST-1 energy range. The joint fit with Fermi-LAT data confirms a preference for a sub-exponential cut-off over a pure exponential, even though both models fail to reproduce the data above several tens of giga-electronvolts. The overall results presented in this paper prove that the LST-1 is an excellent telescope for the observation of pulsars, and improved sensitivity is expected to be achieved with the full CTAO Northern Array.

Galactic transient sources with the Cherenkov Telescope Array Observatory

Monthly Notices of the Royal Astronomical Society · 2025

A wide variety of Galactic sources show transient emission at soft and hard X-ray energies: low- and high-mass X-ray binaries containing compact objects, isolated neutron stars exhibiting extreme variability as magnetars as well as pulsar-wind nebulae. Although most of them can show emission up to MeV and/or GeV energies, many have not yet been detected in the TeV domain by Imaging Atmospheric Cherenkov Telescopes. In this paper, we explore the feasibility of detecting new Galactic transients with the Cherenkov Telescope Array Observatory (CTAO) and the prospects for studying them with Target of Opportunity observations. We show that CTAO will likely detect new sources in the TeV regime, such as the massive microquasars in the Cygnus region, low-mass X-ray binaries with low-viewing angle, flaring emission from the Crab pulsar-wind nebula or other novae explosions, among others. Since some of these sources could also exhibit emission at larger time-scales, we additionally test their detectability at longer exposures. We finally discuss the multiwavelength synergies with other instruments and large astronomical facilities.

Search for Kink Events in Variable Fermi-LAT Blazars

The Astrophysical Journal · 2025

This study explores the detection of quasiperiodic oscillations (QPOs) in blazars as a method to identify kink events within their jets, utilizing both γ-ray and polarized light observations. Focusing on a sample of nine blazars, we analyze γ-ray light curves to identify significant QPOs. In addition to γ-ray data, we incorporated polarized light data corresponding to the same temporal segments to cross-validate the presence of QPOs. However, the limited availability of comprehensive polarized data restricted our ability to perform a thorough analysis across all data sets. Despite these limitations, our analysis reveals a segment where QPOs in polarized light coincided with those observed in γ-rays, providing preliminary evidence supporting the kink origin of these oscillations.

A Two-week IXPE Monitoring Campaign on Mrk 421

The Astrophysical Journal · 2025

X-ray polarization is a unique new probe of the particle acceleration in astrophysical jets made possible through the Imaging X-ray Polarimetry Explorer. Here we report on the first dense X-ray polarization monitoring campaign on the blazar Mrk 421. Our observations were accompanied by an even denser radio and optical polarization campaign. We find significant short-timescale variability in both X-ray polarization degree and angle, including an ∼90° angle rotation about the jet axis. We attribute this to random variations of the magnetic field, consistent with the presence of turbulence but also unlikely to be explained by turbulence alone. At the same time, the degree of lower-energy polarization is significantly lower and shows no more than mild variability. Our campaign provides further evidence for a scenario in which energy-stratified shock-acceleration of relativistic electrons, combined with a turbulent magnetic field, is responsible for optical to X-ray synchrotron emission in blazar jets.

Determining the origin of the X-ray emission in blazars through multiwavelength polarization

Astronomy and Astrophysics · 2025

The origin of the high-energy emission in astrophysical jets from black holes is a highly debated issue. This is particularly true for jets from supermassive black holes, which are among the most powerful particle accelerators in the Universe. So far, the addition of new observations and new messengers have only managed to create more questions than answers. However, the newly available X-ray polarization observations promise to finally distinguish between emission models. We use extensive multiwavelength and polarization campaigns as well as state-of-the-art polarized spectral energy distribution models to attack this problem by focusing on two X-ray polarization observations of blazar BL Lacertae in flaring and quiescent γ-ray states. We find that, regardless of the jet composition and underlying emission model, inverse-Compton scattering from relativistic electrons dominates at X-ray energies.

Multiwavelength study of OT 081: broadband modelling of a transitional blazar

Monthly Notices of the Royal Astronomical Society · 2025

OT 081 is a well-known, luminous blazar that is remarkably variable in many energy bands. We present the first broadband study of the source, which includes very high energy (VHE, <inline-formula><tex-math>$E\gt $</tex-math></inline-formula> 100 GeV) <inline-formula><tex-math>$\gamma$</tex-math></inline-formula>-ray data taken by the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov telescopes) and H.E.S.S. (High Energy Stereoscopic System) imaging Cherenkov telescopes. The discovery of VHE <inline-formula><tex-math>$\gamma$</tex-math></inline-formula>-ray emission happened during a high state of <inline-formula><tex-math>$\gamma$</tex-math></inline-formula>-ray activity in July 2016, observed by many instruments from radio to VHE <inline-formula><tex-math>$\gamma$</tex-math></inline-formula>-rays. We identify four states of activity of the source, one of which includes VHE <inline-formula><tex-math>$\gamma$</tex-math></inline-formula>-ray emission. Variability in the VHE domain is found on daily time-scales. The intrinsic VHE spectrum can be described by a power law with index <inline-formula><tex-math>$3.27\pm 0.44_{\rm stat}\pm 0.15_{\rm sys}$</tex-math></inline-formula> (MAGIC) and <inline-formula><tex-math>$3.39\pm 0.58_{\rm stat}\pm 0.64_{\rm sys}$</tex-math></inline-formula> (H.E.S.S.) in the energy range of 55─300 and 120─500 GeV, respectively. The broadband emission cannot be successfully reproduced by a simple one-zone synchrotron self-Compton model. Instead, an additional external Compton component is required. We test a lepto-hadronic model that reproduces the data set well and a proton-synchrotron-dominated model that requires an extreme proton luminosity. Emission models that are able to successfully represent the data place the emitting region well outside of the broad-line region to a location at which the radiative environment is dominated by the infrared thermal radiation field of the dusty torus. In the scenario described by this flaring activity, the source appears to be a flat spectrum radio quasar (FSRQ), in contrast with past categorizations. This suggests that the source can be considered to be a transitional blazar, intermediate between BL Lac and FSRQ objects.

High Optical-to-X-Ray Polarization Ratio Reveals Compton Scattering in BL Lacertae's Jet

The Astrophysical Journal · 2025

Blazars, supermassive black hole systems with highly relativistic jets aligned with the line of sight, are the most powerful long-lived emitters of electromagnetic emission in the Universe. We report here on a radio-to-gamma-ray multiwavelength campaign on the blazar BL Lacertae with unprecedented polarimetric coverage from radio to X-ray wavelengths. The observations caught an extraordinary event on 2023 November 10─18, when the degree of linear polarization of optical synchrotron radiation reached a record value of 47.5%. In stark contrast, the Imaging X-ray Polarimetry Explorer found that the X-ray (Compton scattering or hadron-induced) emission was polarized at less than 7.4% (3σ confidence level). We argue here that this observational result rules out a hadronic origin of the high-energy emission and strongly favors a leptonic (Compton scattering) origin, thereby breaking the degeneracy between hadronic and leptonic emission models for BL Lacertae and demonstrating the power of multiwavelength polarimetry to address this question. Furthermore, the multiwavelength flux and polarization variability, featuring an extremely prominent rise and decay of the optical polarization degree, is interpreted for the first time by the relaxation of a magnetic "spring" embedded in the newly injected plasma. This suggests that the plasma jet can maintain a predominant toroidal magnetic field component parsecs away from the central engine.

Very high-energy gamma-ray detection and long-term multiwavelength view of the flaring blazar B2 1811+31

Astronomy and Astrophysics · 2025

Context. Among the blazars whose emission has been detected up to very high-energy (VHE; 100 GeV&lt;E&lt;100 TeV) γ rays, intermediate synchrotron-peaked BL Lacs (IBLs) are quite rare. The IBL B2 1811+31 (z = 0.117) exhibited intense flaring activity in 2020. Detailed characterization of the source emission from radio to γ-ray energies was achieved with quasi-simultaneous observations, which led to the first-time detection of VHE γ-ray emission from the source with the MAGIC telescopes. Aims. In this work, we present a comprehensive multiwavelength (MWL) view of B2 1811+31, with a specific focus on the 2020 VHE flare, employing data from MAGIC, Fermi-LAT, Swift-XRT, Swift-UVOT, and several optical and radio ground-based telescopes. Methods. Long-term MWL data were employed to contextualize the high-state episode within the source emissions over 18 years. We investigated the variability, cross-correlations, and classification of the source emissions during low and high states. We propose an interpretative leptonic model for the observed radiative high state. Results. During the 2020 flaring state, the synchrotron peak frequency shifted to higher values and reached the limit of the IBL classification. Variability in timescales of a few hours in the high-energy (HE; 100 MeV&lt;E&lt;100 GeV) γ-ray band poses an upper limit of 6×10<SUP>14</SUP> δ<SUB>D</SUB> cm on the size of the emission region responsible for the γ-ray flare, with δ<SUB>D</SUB> being the relativistic Doppler factor of the region. During the 2020 high state, the average spectrum became harder in the HE γ-ray band compared to the low states. A similar behavior has been observed in X-rays. Conversely, during different activity periods, we find harder-when-brighter trends in X-rays and a hint of softer-when-brighter trends at HE γ rays. A long-term HE γ-ray and optical correlation indicates that the same emission regions dominate the radiative output in both ranges, whereas the evolution at 15 GHz shows no correlation with the fluxes at higher frequencies. We test one-zone and two-zone synchrotron-self-Compton models for describing the broadband spectral energy distribution during the 2020 flaring state and investigate the self-consistency of the proposed scenario.

IOP4

Zenodo · 2025

IOP4, the Interactive Optical Photo-Polarimetric Python Pipeline.

Cosmic ray bubbles from nova super-remnants and their contribution to local cosmic ray spectra

Astronomy and Astrophysics · 2025

Context. Several new phenomena have arisen in the area of study of the repeating thermonuclear explosions called novae. For example, recurrent novae have been proven to be efficient cosmic ray hadronic accelerators thanks to the recent observations of RS Ophiuchi by different γ-ray instruments. Novae have also been demonstrated to have the ability to carve large cavities into the interstellar medium (ISM), with parallels with the remnants of supernovae. Aims. Our aims are to calculate the effects of novae on their surrounding media, and the distances over which these effects dominate over the average quantities that are measured in the ISM. Methods. We calculated the filling factor of novae and their contribution to cosmic ray fluxes using cosmic ray propagation codes. To limit the atomic density of the ISM surrounding the region of RS Oph, we used Fermi-LAT observations of the region. Results. The filling factor of novae in the Galaxy is not significant under all assumptions done in the paper. They do not dominate over the local cosmic ray fluxes, even at the lowest energies, for distances larger than a few parsecs. The particle density of the ISM surrounding them is, however, very much modified, being lowered by more than one order of magnitude with respect to galactic averages, confirming estimates done using other observatories. Conclusions. Even though at global galactic distances, novae do not seem to be dominating cosmic ray transport, they have the power to modify the conditions of their surrounding ISM over parsec distances.

X-Ray Polarization of the High-synchrotron-peak BL Lacertae Object 1ES 1959+650 during Intermediate and High X-Ray Flux States

The Astrophysical Journal · 2025

We report the Imaging X-ray Polarimetry Explorer (IXPE) polarimetric and simultaneous multiwavelength observations of the high-energy-peaked BL Lacertae object (HBL) 1ES 1959+650, performed in 2022 October and 2023 August. In 2022 October, IXPE measured an average polarization degree Π<SUB>X</SUB> = 9.4% ± 1.6% and an electric-vector position angle ψ<SUB>X</SUB> = 53° ± 5°. The polarized X-ray emission can be decomposed into a constant component, plus a rotating component, with the rotation velocity ω<SUB>EVPA</SUB> = (−117 ± 12) deg day<SUP>−1</SUP>. In 2023 August, during a period of pronounced activity of the source, IXPE measured an average Π<SUB>X</SUB> = 12.4% ± 0.7% and ψ<SUB>X</SUB> = 20° ± 2°, with evidence (∼0.4% chance probability) for a rapidly rotating component with ω<SUB>EVPA</SUB> = 1864 ± 34 deg day<SUP>−1</SUP>. These findings suggest the presence of a helical magnetic field in the jet of 1ES 1959+650 or stochastic processes governing the field in turbulent plasma. Our multiwavelength campaigns from radio to X-ray reveal variability in both polarization and flux from optical to X-rays. We interpret the results in terms of a relatively slowly varying component dominating the radio and optical emission, while rapidly variable polarized components dominate the X-ray and provide minor contribution at optical wavelengths. The radio and optical data indicate that on parsec scales the magnetic field is primarily orthogonal to the jet direction. On the contrary, X-ray measurements show a magnetic field almost aligned with the parsec jet direction. Confronting with other IXPE observations, we guess that the magnetic field of HBLs on subparsec scale should be rather unstable, often changing its direction with respect to the Very Long Baseline Array jet.

Detection of RS Oph with LST-1 and modelling of its HE/VHE gamma-ray emission

Astronomy and Astrophysics · 2025

Context. The recurrent nova RS Ophiuchi (RS Oph) underwent a thermonuclear eruption in August 2021. In this event, RS Oph was detected by the High Energy Stereoscopic System (H.E.S.S.), the Major Atmospheric Gamma Imaging Cherenkov (MAGIC), and the first Large-Sized Telescope (LST-1) of the future Cherenkov Telescope Array Observatory (CTAO) at very-high gamma-ray energies above 100 GeV. This means that novae are a new class of very-high-energy (VHE) gamma-ray emitters. Aims. We report the analysis of the RS Oph observations with LST-1. We constrain the particle population that causes the observed emission in hadronic and leptonic scenarios. Additionally, we study the prospects of detecting further novae using LST-1 and the upcoming LST array of CTAO-North. Methods. We conducted target-of-opportunity observations with LST-1 from the first day of this nova event. The data were analysed in the framework of cta-lstchain and Gammapy, the official CTAO-LST reconstruction and analysis packages. One-zone hadronic and leptonic models were considered to model the gamma-ray emission of RS Oph using the spectral information from Fermi-LAT and LST-1, together with public data from the MAGIC and H.E.S.S. telescopes. Results. RS Oph was detected at 6.6σ with LST-1 in the first 6.35 hours of observations following the eruption. The hadronic scenario is preferred over the leptonic scenario considering a proton energy spectrum with a power-law model with an exponential cutoff whose position increases from (0.26 ± 0.08) TeV on day 1 up to (1.6 ± 0.6) TeV on day 4 after the eruption. The deep sensitivity and low energy threshold of the LST-1/LST array will allow us to detect faint novae and increase their discovery rate.

Combined search in dwarf spheroidal galaxies for branon dark matter annihilation signatures with the MAGIC telescopes

Journal of Cosmology and Astroparticle Physics · 2025

Massive brane fluctuations, called branons, behave as weakly interacting massive particles, which is one of the most favored class of candidates to fulfill the role of the dark matter (DM), an elusive kind of matter beyond the Standard Model. We present a multi-target search in dwarf spheroidal galaxies for branon DM annihilation signatures with a total exposure of 354 hours with the ground-based gamma-ray telescope system MAGIC. This search led to the most constraining limits on branon DM in the sub-TeV and multi-TeV DM mass range. Our most stringent limit on the thermally-averaged annihilation cross-section (at 95% confidence level) corresponds to ⟨σv⟩ ≃ 1.9 × 10<SUP>-24</SUP> cm<SUP>3</SUP>s<SUP>-1</SUP> at a branon mass of ∼ 1.5 TeV.

Exploring the capability of the HH 80-81 protostellar jet to accelerate relativistic particles

Astronomy and Astrophysics · 2025

Context. Protostellar jets driven by massive protostars are collimated outflows producing high-speed shocks through dense interstellar medium. Fast shocks can accelerate particles up to relativistic energies via diffusive shock acceleration, producing non-thermal emission that can generate γ-ray photons. HH 80-81 is one of the most powerful collimated protostellar jets in our Galaxy, with non-thermal emission detected in radio, X-ray, and γ-ray bands. Characterizing the γ-ray emission that originates in the accelerated particles of the region is crucial for demonstrating the capability of protostars to accelerate cosmic rays. Aims. Our goal is to determine the particle distribution that is producing the γ-ray spectrum of HH 80-81 in order to ascertain the leptonic or hadronic origin of the γ-ray emission. We aim to associate the high-energy emission in the region with the HH 80-81 system, characterize its spectrum, and elaborate emission models based on what we expect from the diffusive shock acceleration. Methods. We use the 15 yr database provided by the Fermi-LAT satellite to study the high-energy emission of the jet, spanning from 300 MeV to 100 GeV. In addition, we perform a source association based on positional arguments. Then, we employ the naima and Gamera softwares to analyze the possible mechanisms that are producing γ-rays, considering the ambient conditions. We perform a radiative fitting and study the nature of the particles behind the γ-ray emission. Results. By analyzing all the candidates to produce the γ-ray emission that we detect, we conclude that HH 80-81 is the most probable candidate to explain the γ-ray emission in the region. The detected spectrum can be explained by both hadronic and leptonic particle components.

IXPE observation of the low-synchrotron peaked blazar S4 0954+65 during an optical-X-ray flare

Astronomy and Astrophysics · 2025

The X-ray polarization observations, made possible with the Imaging X-ray Polarimetry Explorer (IXPE), offer new ways of probing high-energy emission processes in astrophysical jets from blazars. Here, we report the first X-ray polarization observation of the blazar S4 0954+65 in a high optical and X-ray state. During our multi-wavelength (MWL) campaign of the source, we detected an optical flare whose peak coincided with the peak of an X-ray flare. This optical-X-ray flare most likely took place in a feature moving along the parsec-scale jet, imaged at 43 GHz by the Very Long Baseline Array (VLBA). The 43 GHz polarization angle of the moving component underwent a rotation near the time of the flare. In the optical band, prior to the IXPE observation, we measured the polarization angle to be aligned with the jet axis. In contrast, during the optical flare, the optical polarization angle was perpendicular to the jet axis; after the flare, it reverted to being parallel to the jet axis. Due to the smooth behavior of the optical polarization angle during the flare, we favor shocks as the main acceleration mechanism. We also infer that the ambient magnetic field lines in the jet were parallel to the jet position angle. The average degree of optical polarization during the IXPE observation was (14.3 ± 4.1)%. Despite the flare, we only detected an upper limit of 14% (at 3σ level) on the X-ray polarization degree; however, a reasonable assumption on the X-ray polarization angle results in an upper limit of 8.8% (3σ). We modeled the spectral energy distribution (SED) and spectral polarization distribution (SPD) of S4 0954+65 with leptonic (synchrotron self-Compton) and hadronic (proton and pair synchrotron) models. Our combined MWL polarization observations and SED modeling tentatively disfavor the use of hadronic models for the X-ray emission in S4 0954+65.

cta-observatory/cta-lstchain: v0.10.18 - 2025-02-17

Zenodo · 2025

What's Changed Remove images from list of nodes to be copied (#1350) @moralejo Set info logging level in find_tailcuts (#1349) @moralejo Contributers @moralejo and @vuillaut

Time-dependent modelling of short-term variability in the TeV-blazar VER J0521+211 during the major flare in 2020

Astronomy and Astrophysics · 2025

The BL Lacertae object VER J0521+211 underwent a notable flaring episode in February 2020. A short-term monitoring campaign, led by the MAGIC (Major Atmospheric Gamma Imaging Cherenkov) collaboration, covering a wide energy range from radio to very high-energy (VHE, 100 GeV &lt; E &lt; 100 TeV) gamma rays was organised to study its evolution. These observations resulted in a consistent detection of the source over six consecutive nights in the VHE gamma-ray domain. Combining these nightly observations with an extensive set of multi-wavelength data made modelling of the blazar's spectral energy distribution (SED) possible during the flare. This modelling was performed with a focus on two plausible emission mechanisms: (i) a leptonic two-zone synchrotron-self-Compton scenario, and (ii) a lepto-hadronic one-zone scenario. Both models effectively replicated the observed SED from radio to the VHE gamma-ray band. Furthermore, by introducing a set of evolving parameters, both models were successful in reproducing the evolution of the fluxes measured in different bands throughout the observing campaign. Notably, the lepto-hadronic model predicts enhanced photon and neutrino fluxes at ultra-high energies (E &gt; 100 TeV). While the photon component, generated via decay of neutral pions, is not directly observable as it is subject to intense pair production (and therefore extinction) through interactions with the cosmic microwave background photons, neutrino detectors (e.g. IceCube) can probe the predicted neutrino component. Finally, the analysis of the gamma-ray spectra, observed by MAGIC and the Fermi-LAT telescopes, yielded a conservative 95% confidence upper limit of z ≤ 0.244 for the redshift of this blazar.

Characterization of Markarian 421 during its most violent year: Multiwavelength variability and correlations

Astronomy and Astrophysics · 2025

Aims. Mrk 421 was in its most active state around early 2010, which led to the highest TeV gamma-ray flux ever recorded from any active galactic nuclei (AGN). We aim to characterize the multiwavelength behavior during this exceptional year for Mrk 421, and evaluate whether it is consistent with the picture derived with data from other less exceptional years. Methods. We investigated the period from November 5, 2009, (MJD 55140) until July 3, 2010, (MJD 55380) with extensive coverage from very-high-energy (VHE; E &gt; 100 GeV) gamma rays to radio with MAGIC, VERITAS, Fermi-LAT, RXTE, Swift, GASP-WEBT, VLBA, and a variety of additional optical and radio telescopes. We characterized the variability by deriving fractional variabilities as well as power spectral densities (PSDs). In addition, we investigated images of the jet taken with VLBA and the correlation behavior among different energy bands. Results. Mrk 421 was in widely different states of activity throughout the campaign, ranging from a low-emission state to its highest VHE flux ever recorded. We find the strongest variability in X-rays and VHE gamma rays, and PSDs compatible with power-law functions with indices around 1.5. We observe strong correlations between X-rays and VHE gamma rays at zero time lag with varying characteristics depending on the exact energy band. We also report a marginally significant (∼3σ) positive correlation between high-energy (HE; E &gt; 100 MeV) gamma rays and the ultraviolet band. We detected marginally significant (∼3σ) correlations between the HE and VHE gamma rays, and between HE gamma rays and the X-ray, that disappear when the large flare in February 2010 is excluded from the correlation study, hence indicating the exceptionality of this flaring event in comparison with the rest of the campaign. The 2010 violent activity of Mrk 421 also yielded the first ejection of features in the VLBA images of the jet of Mrk 421. Yet the large uncertainties in the ejection times of these unprecedented radio features prevent us from firmly associating them to the specific flares recorded during the 2010 campaign. We also show that the collected multi-instrument data are consistent with a scenario where the emission is dominated by two regions, a compact and extended zone, which could be considered as a simplified implementation of an energy-stratified jet as suggested by recent IXPE observations.

Gammapy: Python toolbox for gamma-ray astronomy

Zenodo · 2025

Gammapy analyzes gamma-ray data and creates sky images, spectra and lightcurves, from event lists and instrument response information; it can also determine the position, morphology and spectra of gamma-ray sources. It is used to analyze data from H.E.S.S., Fermi-LAT, HAWC, and the Cherenkov Telescope Array (CTA).

Multiband Optical Variability of the Blazar 3C 454.3 on Diverse Timescales

The Astrophysical Journal Supplement Series · 2025

Due to its peculiar and highly variable nature, the blazar 3C 454.3 has been extensively monitored by the WEBT team. Here, we present for the first time these long-term optical flux and color variability results using data acquired in B, V, R, and I bands over a time span of about two decades. We include data from WEBT collaborators and public archives such as SMARTS, Steward Observatory, and Zwicky Transient Facility. The data are binned and segmented to study the source over this long term when more regular sampling was available. During our study, the long-term spectral variability reveals a redder-when-brighter trend, which, however, stabilizes at a particular brightness cutoff of ∼14.5 mag in the I band, after which it saturates and evolves into a complex state. This trend indicates increasing dominance of jet emission over accretion disk (AD) emission until jet emission completely dominates. Plots of the variation in spectral index (following F <SUB> ν </SUB> ∝ ν <SUP>−α </SUP>) reveal a bimodal distribution using a one-day binning. These correlate with two extreme phases of 3C 454.3, an outburst or high-flux state and a quiescent or low-flux state, which are respectively jet- and AD-dominated. We have also conducted intraday variability studies of nine light curves and found that six of them are variable. Discrete correlation function analysis between different pairs of optical wave bands peaks at zero lags, indicating cospatial emission in different optical bands.

Multi-wavelength picture of the misaligned BL Lac object 3C 371

Astronomy and Astrophysics · 2025

Context. The BL Lac object 3C 371 is one of the targets regularly monitored by the Whole-Earth Blazar Telescope (WEBT), a collaboration of observers studying blazar variability on both short and long timescales. Aims. We aim to evaluate the long-term multi-wavelength (MWL) behaviour of 3C 371, comparing it with results derived from its optical emission in our previous study. For this, we make use of the multi-band campaigns organised by the WEBT collaboration in optical and radio between January 2018 and December 2020, and of public data from Swift and Fermi satellites and the MOJAVE Very Large Interferometry programme. Methods. We evaluated the variability shown by the source in each band by quantifying the amplitude variability parameter, and also looked for a possible inter-band correlation using the z-discrete correlation function. We also present a deep analysis of the optical-UV, X-ray, and γ-ray spectral variability. With the MOJAVE data, we performed a kinematics analysis, looking for components propagating along the jet and calculating its kinematics parameters. We then used this set of parameters to interpret the source MWL behaviour, modelling its broadband spectral energy distribution (SED) with theoretical blazar emission scenarios. Results. The MWL variability of the source in the UV, X-ray, and γ-ray bands is comparable to that in optical, especially considering the lower coverage of the first two wavebands. On the other hand, the radio bands show variability of much lower magnitude. Moreover, this MWL emission shows a high degree of correlation, which is compatible with zero lag, again with the exception of the radio emission. The radio VLBI images reveal super-luminal motion of one of the identified components, which we used to set constraints on the jet kinematics and parameters, and to estimate a viewing angle of θ = (9.6 ± 1.6)°, a Doppler factor of δ = 6.0 ± 1.1, and a Lorentz factor of Γ = 6.0 ± 1.8. The polarised radio emission was found to be anti-correlated with the total flux, and to follow the same behaviour as the polarised optical radiation. The optical-UV spectral behaviour shows a mild harder-when-brighter trend on long timescales, and other trends such as redder-when-brighter on shorter timescales. We successfully modelled the broadband emission with a leptonic scenario, where we compared the low and high emission states during the period of complete MWL coverage. The difference between these two states can be ascribed mainly to a hardening of the distribution of particles. The derived features of the source confirm that 3C 371 is a BL Lac whose jet is not well aligned with the line of sight.

Cosmic-ray acceleration and escape from supernova remnant W44 as probed by Fermi-LAT and MAGIC

Astronomy and Astrophysics · 2025

Context. The supernova remnant (SNR) W44 and its surroundings are a prime target for studying the acceleration of cosmic rays (CRs). Several previous studies established an extended gamma-ray emission that is set apart from the radio shell of W44. This emission is thought to originate from escaped high-energy CRs that interact with a surrounding dense molecular cloud complex. Aims. We present a detailed analysis of Fermi-LAT data with an emphasis on the spatial and spectral properties of W44 and its surroundings. We also report the results of the observations performed with the MAGIC telescopes of the northwestern region of W44. Finally, we present an interpretation model to explain the gamma-ray emission of the SNR and its surroundings. Methods. We first performed a detailed spatial analysis of 12 years of Fermi-LAT data at energies above 1 GeV, in order to exploit the better angular resolution, while we set a threshold of 100 MeV for the spectral analysis. We performed a likelihood analysis of 174 hours of MAGIC data above 130 GeV using the spatial information obtained with Fermi-LAT. Results. The combined spectra of Fermi-LAT and MAGIC, extending from 100 MeV to several TeV, were used to derive constraints on the escape of CRs. Using a time-dependent model to describe the particle acceleration and escape from the SNR, we show that the maximum energy of the accelerated particles has to be ≃40 GeV. However, our gamma-ray data suggest that a small number of lower-energy particles also needs to escape. We propose a novel model, the broken-shock scenario, to account for this effect and explain the gamma-ray emission.

Constraints on VHE gamma-ray emission of flat spectrum radio quasars with the MAGIC telescopes

Monthly Notices of the Royal Astronomical Society · 2024

Flat spectrum radio quasars (FSRQs) constitute a class of jetted active galaxies characterized by a very luminous accretion disc, prominent and rapidly moving line-emitting cloud structures (broad-line region, BLR), and a surrounding dense dust structure known as dusty torus. The intense radiation field of the accretion disc strongly determines the observational properties of FSRQs. While hundreds of such sources have been detected at GeV energies, only a handful of them exhibit emission in the very-high-energy (VHE, E<inline-formula><tex-math id="TM0001" notation="LaTeX">$\gtrsim 100$</tex-math></inline-formula> GeV) range. This study presents the results and interpretation derived from a cumulative observation period of 174 h dedicated to nine FSRQs conducted with the Major Atmospheric Gamma-ray Imaging Cherenkov telescopes from 2008 to 2020. Our findings indicate no statistically significant (<inline-formula><tex-math id="TM0002" notation="LaTeX">$\ge$</tex-math></inline-formula>5<inline-formula><tex-math id="TM0003" notation="LaTeX">$\sigma$</tex-math></inline-formula>) signal for any of the studied sources, resulting in upper limits on the emission within the VHE energy range. In two of the sources, we derived quite stringent constraints on the gamma-ray emission in the form of upper limits. Our analysis focuses on modelling the VHE emission of these two sources in search for hints of absorption signatures within the BLR radiation field. For these particular sources, constraints on the distance between the emission region and the central black hole are derived using a phenomenological model. Subsequently, these constraints are tested using a framework based on a leptonic model.

Evidence for a shock-compressed magnetic field in the northwestern rim of Vela Jr. from X-ray polarimetry

Astronomy and Astrophysics · 2024

Synchrotron X-ray emission has been detected from nearly a dozen young supernova remnants (SNRs). X-rays of synchrotron origin exhibit linear polarization in a regular, non-randomly oriented magnetic field. The significant polarized X-ray emission from four such SNRs has already been reported on the basis of observations with the Imaging X-ray Polarimetry Explorer (IXPE). The magnetic-field structure as derived from IXPE observations is radial for Cassiopeia A, Tycho's SNR, and SN 1006, and tangential for RX J1713.7−3946. The latter together with the recent detection of a tangential magnetic field in SNR 1E 0102.2-7219 by the Australia Telescope Compact Array in the radio band shows that tangential magnetic fields can also be present in young SNRs. Thus, the dichotomy in polarization between young and middle-aged SNRs (radial magnetic fields in young SNRs, but tangential magnetic fields in middle-aged SNRs), previously noticed in the radio band, deserves additional attention. The present analysis of IXPE observations determines, for the first time, a magnetic-field structure in the northwestern rim of Vela Jr, also known as RX J0852.0−4622, and provides a new example of a young SNR with a tangential magnetic field.

A wiggling filamentary jet at the origin of the blazar multi-wavelength behaviour

Astronomy and Astrophysics · 2024

Context. Blazars are beamed active galactic nuclei (AGNs) known for their strong multi-wavelength variability on timescales ranging from years down to minutes. Many different models have been proposed to explain this variability. Aims. We aim to investigate the suitability of the twisting jet model presented in previous works to explain the multi-wavelength behaviour of BL Lacertae, the prototype of one of the blazar classes. According to this model, the jet is inhomogeneous, curved, and twisting, and the long-term variability is due to changes in the Doppler factor due to variations in the orientation of the jet-emitting regions. Methods. We analysed optical data of the source obtained during monitoring campaigns organised by the Whole Earth Blazar Telescope (WEBT) in 2019─2022, together with radio data from the WEBT and other teams, and γ-ray data from the Fermi satellite. In this period, BL Lacertae underwent an extraordinary activity phase, reaching its historical optical and γ-ray brightness maxima. Results. The application of the twisting jet model to the source light curves allows us to infer the wiggling motion of the optical, radio, and γ-ray jet-emitting regions. The optical-radio correlation shows that the changes in the radio viewing angle follow those in the optical viewing angle by about 120 days, and it suggests that the jet is composed of plasma filaments, which is in agreement with some radio high-resolution observations of other sources. The γ-ray emitting region is found to be co-spatial with the optical one, and the analysis of the γ-optical correlation is consistent with both the geometric interpretation and a synchrotron self-Compton (SSC) origin of the high-energy photons. Conclusions. We propose a geometric scenario where the jet is made up of a pair of emitting plasma filaments in a sort of double-helix curved rotating structure, whose wiggling motion produces changes in the Doppler beaming and can thus explain the observed multi-wavelength long-term variability.

Broadband multi-wavelength properties of M87 during the 2018 EHT campaign including a very high energy flaring episode

Astronomy and Astrophysics · 2024

Context. The nearby elliptical galaxy M87 contains one of only two supermassive black holes whose emission surrounding the event horizon has been imaged by the Event Horizon Telescope (EHT). In 2018, more than two dozen multi-wavelength (MWL) facilities (from radio to γ-ray energies) took part in the second M87 EHT campaign. Aims. The goal of this extensive MWL campaign was to better understand the physics of the accreting black hole M87*, the relationship between the inflow and inner jets, and the high-energy particle acceleration. Understanding the complex astrophysics is also a necessary first step towards performing further tests of general relativity. Methods. The MWL campaign took place in April 2018, overlapping with the EHT M87* observations. We present a new, contemporaneous spectral energy distribution (SED) ranging from radio to very high-energy (VHE) γ-rays as well as details of the individual observations and light curves. We also conducted phenomenological modelling to investigate the basic source properties. Results. We present the first VHE γ-ray flare from M87 detected since 2010. The flux above 350 GeV more than doubled within a period of ≍36 hours. We find that the X-ray flux is enhanced by about a factor of two compared to 2017, while the radio and millimetre core fluxes are consistent between 2017 and 2018. We detect evidence for a monotonically increasing jet position angle that corresponds to variations in the bright spot of the EHT image. Conclusions. Our results show the value of continued MWL monitoring together with precision imaging for addressing the origins of high-energy particle acceleration. While we cannot currently pinpoint the precise location where such acceleration takes place, the new VHE γ-ray flare already presents a challenge to simple one-zone leptonic emission model approaches, and it emphasises the need for combined image and spectral modelling.

A new method of reconstructing images of gamma-ray telescopes applied to the LST-1 of CTAO

Astronomy and Astrophysics · 2024

Context. Imaging atmospheric Cherenkov telescopes (IACTs) are used to observe very high-energy photons from the ground. Gamma rays are indirectly detected through the Cherenkov light emitted by the air showers they induce. The new generation of experiments, in particular the Cherenkov Telescope Array Observatory (CTAO), sets ambitious goals for discoveries of new gamma-ray sources and precise measurements of the already discovered ones. To achieve these goals, both hardware and data analysis must employ cuttingedge techniques. This also applies to the LST-1, the first IACT built for the CTAO, which is currently taking data on the Canary island of La Palma. Aims. This paper introduces a new event reconstruction technique for IACT data, aiming to improve the image reconstruction quality and the discrimination between the signal and the background from misidentified hadrons and electrons. Methods. The technique models the development of the extensive air shower signal, recorded as a waveform per pixel, seen by CTAO telescopes' cameras. Model parameters are subsequently passed to random forest regressors and classifiers to extract information on the primary particle. Results. The new reconstruction was applied to simulated data and to data from observations of the Crab Nebula performed by the LST-1. The event reconstruction method presented here shows promising performance improvements. The angular and energy resolution, and the sensitivity, are improved by 10 to 20% over most of the energy range. At low energy, improvements reach up to 22%, 47%, and 50%, respectively. A future extension of the method to stereoscopic analysis for telescope arrays will be the next important step.

Probing the polarized emission from SMC X-1: The brightest X-ray pulsar observed by IXPE

Astronomy and Astrophysics · 2024

Recent observations of X-ray pulsars (XRPs) performed by the Imaging X-ray Polarimetry Explorer (IXPE) have made it possible to investigate the intricate details of these objects in a new way, thanks to the added value of X-ray polarimetry. Here we present the results of the IXPE observations of SMC X-1, a member of the small group of XRPs displaying super-orbital variability. SMC X-1 was observed by IXPE three separate times during the high state of its super-orbital period. The observed luminosity in the 2─8 keV energy band of L ∼ 2 × 10<SUP>38</SUP> erg s<SUP>−1</SUP> makes SMC X-1 the brightest XRP ever observed by IXPE. We detect significant polarization in all three observations, with values of the phase-averaged polarization degree (PD) and polarization angle (PA) of 3.2 ± 0.8% and 97° ±8° for Observation 1, 3.0 ± 0.9% and 90° ±8° for Observation 2, and 5.5 ± 1.1% and 80° ±6° for Observation 3, for the spectro-polarimetric analysis. The observed PD shows an increase over time with decreasing luminosity, while the PA decreases in decrements of ∼10°. The phase-resolved spectro-polarimetric analysis reveals significant detection of polarization in three out of seven phase bins, with the PD ranging between ∼2% and ∼10%, and a corresponding range in the PA from ∼70° to ∼100°. The pulse-phase resolved PD displays an apparent anti-correlation with the flux. Using the rotating vector model, we obtain constraints on the pulsar's geometrical properties for the individual observations. The position angle of the pulsar displays an evolution over time supporting the idea that we observe changes related to different super-orbital phases. Scattering in the wind of the precessing accretion disk may be responsible for the behavior of the polarimetric properties observed during the high-state of SMC X-1's super-orbital period.

Standardised formats and open-source analysis tools for the MAGIC telescopes data

Journal of High Energy Astrophysics · 2024

Instruments for gamma-ray astronomy at Very High Energies (<mml:math><mml:mi>E</mml:mi><mml:mo>&gt;</mml:mo><mml:mn>100</mml:mn><mml:mspace></mml:mspace><mml:mrow><mml:mi>GeV</mml:mi></mml:mrow></mml:math>) have traditionally derived their scientific results through proprietary data and software. Data standardisation has become a prominent issue in this field both as a requirement for the dissemination of data from the next generation of gamma-ray observatories and as an effective solution to realise public data legacies of current-generation instruments. Specifications for a standardised gamma-ray data format have been proposed as a community effort and have already been successfully adopted by several instruments. We present the first production of standardised data from the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes. We converted <mml:math><mml:mn>166</mml:mn><mml:mspace></mml:mspace><mml:mi>h</mml:mi></mml:math> of observations from different sources and validated their analysis with the open-source software Gammapy. We consider six data sets representing different scientific and technical analysis cases and compare the results obtained analysing the standardised data with open-source software against those produced with the MAGIC proprietary data and software. Aiming at a systematic production of MAGIC data in this standardised format, we also present the implementation of a database-driven pipeline automatically performing the MAGIC data reduction from the calibrated down to the standardised data level. In all the cases selected for the validation, we obtain results compatible with the MAGIC proprietary software, both for the manual and for the automatic data productions. Part of the validation data set is also made publicly available, thus representing the first large public release of MAGIC data. This effort and this first data release represent a technical milestone toward the realisation of a public MAGIC data legacy.

Studying geometry of the ultraluminous X-ray pulsar Swift J0243.6+6124 using X-ray and optical polarimetry

Astronomy and Astrophysics · 2024

Discovery of pulsations from a number of ultra-luminous X-ray (ULX) sources proved that accretion onto neutron stars can produce luminosities exceeding the Eddington limit by several orders of magnitude. The conditions necessary to achieve such high luminosities as well as the exact geometry of the accretion flow in the neutron star vicinity are, however, a matter of debate. The pulse phase-resolved polarization measurements that became possible with the launch of the Imaging X-ray Polarimetry Explorer (IXPE) can be used to determine the pulsar geometry and its orientation relative to the orbital plane. They provide an avenue to test different theoretical models of ULX pulsars. In this paper we present the results of three IXPE observations of the first Galactic ULX pulsar Swift J0243.6+6124 during its 2023 outburst. We find strong variations in the polarization characteristics with the pulsar phase. The average polarization degree increases from about 5% to 15% as the flux dropped by a factor of three in the course of the outburst. The polarization angle (PA) as a function of the pulsar phase shows two peaks in the first two observations, but changes to a characteristic sawtooth pattern in the remaining data set. This is not consistent with a simple rotating vector model. Assuming the existence of an additional constant polarized component, we were able to fit the three observations with a common rotating vector model and obtain constraints on the pulsar geometry. In particular, we find the pulsar angular momentum inclination with respect to the line of sight of i<SUB>p</SUB> = 15°─40°, the magnetic obliquity of θ<SUB>p</SUB> = 60°─80°, and the pulsar spin position angle of χ<SUB>p</SUB> ≍ −50°, which significantly differs from the constant component PA of about 10°. Combining these X-ray measurements with the optical PA, we find evidence for at least a 30° misalignment between the pulsar angular momentum and the binary orbital axis.

Prospects for a survey of the galactic plane with the Cherenkov Telescope Array

Journal of Cosmology and Astroparticle Physics · 2024

Approximately one hundred sources of very-high-energy (VHE) gamma rays are known in the Milky Way, detected with a combination of targeted observations and surveys. A survey of the entire Galactic Plane in the energy range from a few tens of GeV to a few hundred TeV has been proposed as a Key Science Project for the upcoming Cherenkov Telescope Array Observatory (CTAO). This article presents the status of the studies towards the Galactic Plane Survey (GPS). We build and make publicly available a sky model that combines data from recent observations of known gamma-ray emitters with state-of-the-art physically-driven models of synthetic populations of the three main classes of established Galactic VHE sources (pulsar wind nebulae, young and interacting supernova remnants, and compact binary systems), as well as of interstellar emission from cosmic-ray interactions in the Milky Way. We also perform an optimisation of the observation strategy (pointing pattern and scheduling) based on recent estimations of the instrument performance. We use the improved sky model and observation strategy to simulate GPS data corresponding to a total observation time of 1620 hours spread over ten years. Data are then analysed using the methods and software tools under development for real data. Under our model assumptions and for the realisation considered, we show that the GPS has the potential to increase the number of known Galactic VHE emitters by almost a factor of five. This corresponds to the detection of more than two hundred pulsar wind nebulae and a few tens of supernova remnants at average integral fluxes one order of magnitude lower than in the existing sample above 1 TeV, therefore opening the possibility to perform unprecedented population studies. The GPS also has the potential to provide new VHE detections of binary systems and pulsars, to confirm the existence of a hypothetical population of gamma-ray pulsars with an additional TeV emission component, and to detect bright sources capable of accelerating particles to PeV energies (PeVatrons). Furthermore, the GPS will constitute a pathfinder for deeper follow-up observations of these source classes. Finally, we show that we can extract from GPS data an estimate of the contribution to diffuse emission from unresolved sources, and that there are good prospects of detecting interstellar emission and statistically distinguishing different scenarios. Thus, a survey of the entire Galactic plane carried out from both hemispheres with CTAO will ensure a transformational advance in our knowledge of Galactic VHE source populations and interstellar emission.

Prospects for γ-ray observations of the Perseus galaxy cluster with the Cherenkov Telescope Array

Journal of Cosmology and Astroparticle Physics · 2024

Galaxy clusters are expected to be both dark matter (DM) reservoirs and storage rooms for the cosmic-ray protons (CRp) that accumulate along the cluster's formation history. Accordingly, they are excellent targets to search for signals of DM annihilation and decay at γ-ray energies and are predicted to be sources of large-scale γ-ray emission due to hadronic interactions in the intracluster medium (ICM). In this paper, we estimate the sensitivity of the Cherenkov Telescope Array (CTA) to detect diffuse γ-ray emission from the Perseus galaxy cluster. We first perform a detailed spatial and spectral modelling of the expected signal for both the DM and the CRp components. For each case, we compute the expected CTA sensitivity accounting for the CTA instrument response functions. The CTA observing strategy of the Perseus cluster is also discussed. In the absence of a diffuse signal (non-detection), CTA should constrain the CRp to thermal energy ratio X <SUB>500</SUB> within the characteristic radius R <SUB>500</SUB> down to about X <SUB>500</SUB> &lt; 3 × 10<SUP>-3</SUP>, for a spatial CRp distribution that follows the thermal gas and a CRp spectral index α<SUB>CRp</SUB> = 2.3. Under the optimistic assumption of a pure hadronic origin of the Perseus radio mini-halo and depending on the assumed magnetic field profile, CTA should measure α<SUB>CRp</SUB> down to about ∆α<SUB>CRp</SUB> ≃ 0.1 and the CRp spatial distribution with 10% precision, respectively. Regarding DM, CTA should improve the current ground-based γ-ray DM limits from clusters observations on the velocity-averaged annihilation cross-section by a factor of up to ∼ 5, depending on the modelling of DM halo substructure. In the case of decay of DM particles, CTA will explore a new region of the parameter space, reaching models with τ <SUB>χ</SUB> &gt; 10<SUP>27</SUP> s for DM masses above 1 TeV. These constraints will provide unprecedented sensitivity to the physics of both CRp acceleration and transport at cluster scale and to TeV DM particle models, especially in the decay scenario.

Analysis of Crab X-Ray Polarization Using Deeper Imaging X-Ray Polarimetry Explorer Observations

The Astrophysical Journal · 2024

We present Crab X-ray polarization measurements using Imaging X-Ray Polarimetry Explorer (IXPE) data with a total exposure of 300 ks, three times more than the initial 2022 discovery paper. Polarization is detected in three times more pulsar phase bins, revealing an S-shaped +40° polarization angle sweep in the main pulse and &gt;1σ departures from the OPTIMA optical polarization in both pulses, suggesting different radiation mechanisms or sites for the polarized emission at the two wavebands. Our polarization map of the inner nebula reveals a toroidal magnetic field, as seen in prior IXPE analyses. Along the southern jet, the magnetic field orientation relative to the jet axis changes from perpendicular to parallel and the polarization degree decreases by ∼6%. These observations may be explained by kink instabilities along the jet or a collision with a dense, jet-deflecting medium at the tip. Using spectropolarimetric analysis, we find asymmetric polarization in the four quadrants of the inner nebula, as expected for a toroidal field geometry, and a spatial correlation between polarization degree and photon index.

A detailed study of the very high-energy Crab pulsar emission with the LST-1

Astronomy and Astrophysics · 2024

Context. To date, three pulsars have been firmly detected by imaging atmospheric Cherenkov telescopes (IACTs). Two of them reached the TeV energy range, challenging models of very high-energy (VHE) emission in pulsars. More precise observations are needed to better characterize pulsar emission at these energies. The LST-1 is the prototype of the large-sized telescopes, which will be part of the Cherenkov Telescope Array Observatory (CTAO). Its improved performance over previous IACTs makes it well suited for studying pulsars. Aims. In this work we study the Crab pulsar emission with the LST-1, improving upon and complementing the results from other telescopes. Crab pulsar observations can also be used to characterize the potential of the LST-1 to study other pulsars and detect new ones. Methods. We analyzed a total of ∼103 hours of gamma-ray observations of the Crab pulsar conducted with the LST-1 in the period from September 2020 to January 2023. The observations were carried out at zenith angles of less than 50 degrees. To characterize the Crab pulsar emission over a broader energy range, a new analysis of the Fermi/LAT data, including ∼14 years of observations, was also performed. Results. The Crab pulsar phaseogram, long-term light curve, and phase-resolved spectra are reconstructed with the LST-1 from 20 GeV to 450 GeV for the first peak and up to 700 GeV for the second peak The pulsed emission is detected with a significance level of 15.2σ. The two characteristic emission peaks of the Crab pulsar are clearly detected (&gt; 10σ), as is the so-called bridge emission between them (5.7σ). We find that both peaks are described well by power laws, with spectral indices of ∼3.44 and ∼3.03, respectively. The joint analysis of Fermi/LAT and LST-1 data shows a good agreement between the two instruments in their overlapping energy range. The detailed results obtained from the first observations of the Crab pulsar with the LST-1 show the potential that CTAO will have to study this type of source.

X-Ray and Multiwavelength Polarization of Mrk 501 from 2022 to 2023

The Astrophysical Journal · 2024

We present multiwavelength polarization measurements of the luminous blazar Mrk 501 over a 14 month period. The 2─8 keV X-ray polarization was measured with the Imaging X-ray Polarimetry Explorer (IXPE) with six 100 ks observations spanning from 2022 March to 2023 April. Each IXPE observation was accompanied by simultaneous X-ray data from NuSTAR, Swift/XRT, and/or XMM-Newton. Complementary optical─infrared polarization measurements were also available in the B, V, R, I, and J bands, as were radio polarization measurements from 4.85 GHz to 225.5 GHz. Among the first five IXPE observations, we did not find significant variability in the X-ray polarization degree and angle with IXPE. However, the most recent sixth observation found an elevated polarization degree at &gt;3σ above the average of the other five observations. The optical and radio measurements show no apparent correlations with the X-ray polarization properties. Throughout the six IXPE observations, the X-ray polarization degree remained higher than, or similar to, the R-band optical polarization degree, which remained higher than the radio value. This is consistent with the energy-stratified shock scenario proposed to explain the first two IXPE observations, in which the polarized X-ray, optical, and radio emission arises from different regions.

IXPE observation of PKS 2155─304 reveals the most highly polarized blazar

Astronomy and Astrophysics · 2024

We report the X-ray polarization properties of the high-synchrotron-peaked (HSP) blazar PKS 2155−304 based on observations with the Imaging X-ray Polarimetry Explorer (IXPE). We observed the source between Oct 27 and Nov 7, 2023. We also conducted an extensive contemporaneous multiwavelength (MW) campaign. We find that during the first half (T<SUB>1</SUB>) of the IXPE pointing, the source exhibited the highest X-ray polarization degree detected for an HSP blazar thus far, (30.7 ± 2.0)%; this dropped to (15.3 ± 2.1)% during the second half (T<SUB>2</SUB>). The X-ray polarization angle remained stable during the IXPE pointing at 129.4° ±1.8° and 125.4° ±3.9° during T<SUB>1</SUB> and T<SUB>2</SUB>, respectively. Meanwhile, the optical polarization degree remained stable during the IXPE pointing, with average host-galaxy-corrected values of (4.3 ± 0.7)% and (3.8 ± 0.9)% during the T<SUB>1</SUB> and T<SUB>2</SUB>, respectively. During the IXPE pointing, the optical polarization angle changed achromatically from ∼140° to ∼90° and back to ∼130°. Despite several attempts, we only detected (99.7% conf.) the radio polarization once (during T<SUB>2</SUB>, at 225.5 GHz): with degree (1.7 ± 0.4)% and angle 112.5° ±5.5°. The direction of the broad pc-scale jet is rather ambiguous and has been found to point to the east and south at different epochs; however, on larger scales (&gt; 1.5 pc) the jet points toward the southeast (∼135°), similarly to all of the MW polarization angles. Moreover, the X-ray-to-optical polarization degree ratios of ∼7 and ∼4 during T<SUB>1</SUB> and T<SUB>2</SUB>, respectively, are similar to previous IXPE results for several HSP blazars. These findings, combined with the lack of correlation of temporal variability between the MW polarization properties, agree with an energy-stratified shock-acceleration scenario in HSP blazars.

The flaring activity of blazar AO 0235+164 in 2021

Astronomy and Astrophysics · 2024

Context. The blazar AO 0235+164, located at redshift z = 0.94, has displayed interesting and repeating flaring activity in the past, with recent episodes in 2008 and 2015. In 2020, the source brightened again, starting a new flaring episode that peaked in 2021. Aims. We study the origin and properties of the 2021 flare in relation to previous studies and the historical behavior of the source, in particular the 2008 and 2015 flaring episodes. Methods.We analyzed the multiwavelength photo-polarimetric evolution of the source. From Very Long Baseline Array images, we derived the kinematic parameters of new components associated with the 2021 flare. We used this information to constrain a model for the spectral energy distribution of the emission during the flaring period. We propose an analytical geometric model to test whether the observed wobbling of the jet is consistent with precession. Results. We report the appearance of two new components that are ejected in a different direction than previously, confirming the wobbling of the jet. We find that the direction of ejection is consistent with that of a precessing jet. Our derived period agrees with the values commonly found in the literature. Modeling of the spectral energy distribution further confirms that the differences between flares can be attributed to geometrical effects.

X-ray polarization measurement of the gold standard of radio-quiet active galactic nuclei: NGC 1068

Astronomy and Astrophysics · 2024

Context. NGC 1068 is the most observed radio-quiet active galactic nucleus (AGN) in polarimetry, yet its high-energy polarization has never been probed before due to a lack of dedicated polarimeters. Aims. Using the first X-ray polarimeter sensitive enough to measure the polarization of AGNs, we want to probe the orientation and geometric arrangement of (sub)parsec-scale matter around the X-ray source. Methods. We used the Imaging X-ray Polarimetry Explorer (IXPE) satellite to measure, for the first time, the 2─8 keV polarization of NGC 1068. We pointed IXPE at the target for a net exposure time of 1.15 Ms, in addition to using two Chandra snapshots of ∼10 ks each in order to account for the potential impact of several ultraluminous X-ray sources (ULXs) within IXPE's field of view. Results. We measured a 2─8 keV polarization degree of 12.4% ± 3.6% and an electric vector polarization angle of 101° ± 8° at a 68% confidence level. If we exclude the spectral region containing bright Fe K lines and other soft X-ray lines where depolarization occurs, the polarization fraction rises to 21.3% ± 6.7% in the 3.5─6.0 keV band, with a similar polarization angle. The observed polarization angle is found to be perpendicular to the parsec-scale radio jet. Using a combined Chandra and IXPE analysis plus multiwavelength constraints, we estimated that the circumnuclear "torus" may sustain a half-opening angle of 50─55° (from the vertical axis of the system). Conclusions. Thanks to IXPE, we have measured the X-ray polarization of NGC 1068 and found comparable results, both in terms of the polarization angle orientation with respect to the radio jet and the torus half-opening angle, to the X-ray polarimetric measurement achieved for the other archetypal Compton-thick AGN: the Circinus galaxy. Probing the geometric arrangement of parsec-scale matter in extragalactic objects is now feasible thanks to X-ray polarimetry.

Observations of Low and Intermediate Spectral Peak Blazars with the Imaging X-Ray Polarimetry Explorer

The Astrophysical Journal · 2024

We present X-ray polarimetry observations from the Imaging X-ray Polarimetry Explorer (IXPE) of three low spectral peak and one intermediate spectral peak blazars, namely 3C 273, 3C 279, 3C 454.3, and S5 0716+714. For none of these objects was IXPE able to detect X-ray polarization at the 3σ level. However, we placed upper limits on the polarization degree at ∼10%─30%. The undetected polarizations favor models where the X-ray band is dominated by unpolarized photons upscattered by relativistic electrons in the jets of blazars, although hadronic models are not completely eliminated. We discuss the X-ray polarization upper limits in the context of our contemporaneous multiwavelength polarization campaigns.

Testing particle acceleration in blazar jets with continuous high-cadence optical polarization observations

Astronomy and Astrophysics · 2024

Variability can be the pathway to understanding the physical processes in astrophysical jets. However, the high-cadence observations required to test particle acceleration models are still missing. Here we report on the first attempt to produce continuous, &gt; 24 hour polarization light curves of blazars using telescopes distributed across the globe, following the rotation of the Earth, to avoid the rising Sun. Our campaign involved 16 telescopes in Asia, Europe, and North America. We observed BL Lacertae and CGRaBS J0211+1051 for a combined 685 telescope hours. We find large variations in the polarization degree and angle for both sources on sub-hour timescales as well as a ∼180° rotation of the polarization angle in CGRaBS J0211+1051 in less than two days. We compared our high-cadence observations to particle-in-cell magnetic reconnection and turbulent plasma simulations. We find that although the state-of-the-art simulation frameworks can produce a large fraction of the polarization properties, they do not account for the entirety of the observed polarization behavior in blazar jets.

Complex rotational dynamics of the neutron star in Hercules X-1 revealed by X-ray polarization

Nature Astronomy · 2024

In an accreting X-ray pulsar, a neutron star accretes matter from a companion star through an accretion disk. The magnetic field of the rotating neutron star disrupts the inner edge of the disk, funnelling the gas to flow onto the poles on its surface. Hercules X-1 is a prototypical persistent X-ray pulsar about 7 kpc from Earth. Its emission varies on three distinct timescales: the neutron star rotates every 1.2 s, it is eclipsed by its companion each 1.7 d, and the system exhibits a superorbital period of 35 d, which has remained stable since its discovery. Several lines of evidence point to the source of this variation as the precession of the accretion disk or that of the neutron star. Despite the many hints over the past 50 yr, the precession of the neutron star itself has yet not been confirmed or refuted. X-ray polarization measurements (probing the spin geometry of Her X-1) with the Imaging X-ray Polarimetry Explorer suggest that free precession of the neutron star crust sets the 35 d period; this has the important implication that its crust is somewhat asymmetric by a few parts per ten million.

IOP4, the Interactive Optical Photo-Polarimetric Python Pipeline

The Astronomical Journal · 2024

IOP4 is a pipeline to perform photometry and polarimetry analysis of optical data from Calar Alto (CAHA) and Sierra Nevada (OSN) observatories. IOP4 implements Object Relational Mapping to seamlessly integrate all information about the reduction and results in a database that can be used to query and plot results, flag data, and inspect the reduction process in an integrated fashion with the whole pipeline. It also ships with an already built-in web interface that can be used out of the box to browse the database and supervise all pipeline processes. It is built to ease debugging and inspection of data. Reduction from five different instruments are already implemented: RoperT90, AndorT90, DIPOL (at OSN 0.9 m telescope), AndorT150 (OSN 1.5 m telescope), and CAFOS (CAHA 2.2 m telescope). IOP4's modular design allows for easy integration of new observatories and instruments, and its results have already featured in several high-impact refereed publications. In this paper we describe the implementation and characteristics of IOP4.

Cygnus X-3 revealed as a Galactic ultraluminous X-ray source by IXPE

Nature Astronomy · 2024

The accretion of matter by compact objects can be inhibited by radiation pressure if the luminosity exceeds a critical value known as the Eddington limit. The discovery of ultraluminous X-ray sources has shown that accretion can proceed even when the apparent luminosity considerably exceeds this limit. A high apparent luminosity might be produced due to the geometric beaming of radiation by an outflow. The outflow half-opening angle, which determines the amplification due to beaming, has never been robustly constrained. Using the Imaging X-ray Polarimetry Explorer, we measured the X-ray polarization in the Galactic X-ray binary Cygnus X-3 (Cyg X-3). We found high, &gt;20%, nearly energy-independent linear polarization orthogonal to the direction of the radio ejections. These properties unambiguously indicate the presence of a collimating outflow from the X-ray binary Cyg X-3 and constrain its half-opening angle to ≲15°. Thus, the source can be used as a laboratory for studying the supercritical accretion regime. This finding underscores the importance of X-ray polarimetry in advancing our understanding of accreting sources.

Discovery of a strong rotation of the X-ray polarization angle in the galactic burster GX 13+1

Astronomy and Astrophysics · 2024

Weakly magnetized neutron stars in X-ray binaries show a complex phenomenology with several spectral components that can be associated with the accretion disk, the boundary, and/or a spreading layer, a corona, and a wind. Spectroscopic information alone, however, is not enough to distinguish these components. The analysis of the timing data revealed that most of the variability, and in particular, kilohertz quasi-period oscillations, are associated with the high-energy component that corresponds to the boundary and/or spreading layer. Additional information about the nature of the spectral components, and in particular, about the geometry of the emission region, can be provided by X-ray polarimetry. One of the objects of the class, a bright, persistent, and rather peculiar galactic Type I X-ray burster GX 13+1, was observed with the Imaging X-ray Polarimetry Explorer (IXPE) and the XMM-Newton. Using the XMM-Newton data, we obtained the best-fit values for the continuum spectral parameters and detected strong absorption lines associated with the accretion disk wind. IXPE data showed the source to be significantly polarized in the 2-8 keV energy band, with an overall polarization degree (PD) of 1.4%±0.3% at a polarization angle (PA) of ‒2° ±6° (errors at the 68% confidence level). During the two-day long observation, we detected rotation of the PA by about 70° with the corresponding changes in the PD from 2% to nondetectable and then up to 5%. These variations in polarization properties are not accompanied by visible spectral state changes of the source. The energy-resolved polarimetric analysis showed a significant change in polarization, from being strongly dependent on energy at the beginning of the observation to being almost constant with energy in the later parts of the observation. As a possible interpretation, we suggest a constant polarization component, strong wind scattering, or a different polarization of the two main spectral components with an individually peculiar behavior. The rotation of the PA suggests a misalignment of the neutron star spin from the orbital axis.

Dark matter line searches with the Cherenkov Telescope Array

Journal of Cosmology and Astroparticle Physics · 2024

Monochromatic gamma-ray signals constitute a potential smoking gun signature for annihilating or decaying dark matter particles that could relatively easily be distinguished from astrophysical or instrumental backgrounds. We provide an updated assessment of the sensitivity of the Cherenkov Telescope Array (CTA) to such signals, based on observations of the Galactic centre region as well as of selected dwarf spheroidal galaxies. We find that current limits and detection prospects for dark matter masses above 300 GeV will be significantly improved, by up to an order of magnitude in the multi-TeV range. This demonstrates that CTA will set a new standard for gamma-ray astronomy also in this respect, as the world's largest and most sensitive high-energy gamma-ray observatory, in particular due to its exquisite energy resolution at TeV energies and the adopted observational strategy focussing on regions with large dark matter densities. Throughout our analysis, we use up-to-date instrument response functions, and we thoroughly model the effect of instrumental systematic uncertainties in our statistical treatment. We further present results for other potential signatures with sharp spectral features, e.g. box-shaped spectra, that would likewise very clearly point to a particle dark matter origin.

An IXPE-led X-Ray Spectropolarimetric Campaign on the Soft State of Cygnus X-1: X-Ray Polarimetric Evidence for Strong Gravitational Lensing

The Astrophysical Journal · 2024

We present the first X-ray spectropolarimetric results for Cygnus X-1 in its soft state from a campaign of five IXPE observations conducted during 2023 May─June. Companion multiwavelength data during the campaign are likewise shown. The 2─8 keV X-rays exhibit a net polarization degree PD = 1.99% ± 0.13% (68% confidence). The polarization signal is found to increase with energy across the Imaging X-ray Polarimetry Explorer's (IXPE) 2─8 keV bandpass. The polarized X-rays exhibit an energy-independent polarization angle of PA = −25.°7 ± 1.°8 east of north (68% confidence). This is consistent with being aligned to Cyg X-1's au-scale compact radio jet and its parsec-scale radio lobes. In comparison to earlier hard-state observations, the soft state exhibits a factor of 2 lower polarization degree but a similar trend with energy and a similar (also energy-independent) position angle. When scaling by the natural unit of the disk temperature, we find the appearance of a consistent trend line in the polarization degree between the soft and hard states. Our favored polarimetric model indicates that Cyg X-1's spin is likely high (a <SUB>*</SUB> ≳ 0.96). The substantial X-ray polarization in Cyg X-1's soft state is most readily explained as resulting from a large portion of X-rays emitted from the disk returning and reflecting off the disk surface, generating a high polarization degree and a polarization direction parallel to the black hole spin axis and radio jet. In IXPE's bandpass, the polarization signal is dominated by the returning reflection emission. This constitutes polarimetric evidence for strong gravitational lensing of X-rays close to the black hole.

Constraints on Lorentz invariance violation from the extraordinary Mrk 421 flare of 2014 using a novel analysis method

Journal of Cosmology and Astroparticle Physics · 2024

The Lorentz Invariance Violation (LIV), a proposed consequence of certain quantum gravity (QG) scenarios, could instigate an energy-dependent group velocity for ultra-relativistic particles. This energy dependence, although suppressed by the massive QG energy scale E_QG, expected to be on the level of the Planck energy 1.22 × 10<SUP>19</SUP> GeV, is potentially detectable in astrophysical observations. In this scenario, the cosmological distances traversed by photons act as an amplifier for this effect. By leveraging the observation of a remarkable flare from the blazar Mrk 421, recorded at energies above 100 GeV by the MAGIC telescopes on the night of April 25 to 26, 2014, we look for time delays scaling linearly and quadratically with the photon energies. Using for the first time in LIV studies a binned-likelihood approach we set constraints on the QG energy scale. For the linear scenario, we set 95% lower limits E_QG&gt;2.7×10<SUP>17</SUP> GeV for the subluminal case and E_QG&gt; 3.6 ×10<SUP>17</SUP> GeV for the superluminal case. For the quadratic scenario, the 95% lower limits for the subluminal and superluminal cases are E_QG&gt;2.6 ×10<SUP>10</SUP> GeV and E_QG&gt;2.5×10<SUP>10</SUP> GeV, respectively.

IOP4

Zenodo · 2024

IOP4, the Interactive Optical Photo-Polarimetric Python Pipeline.

Tracking the X-Ray Polarization of the Black Hole Transient Swift J1727.8─1613 during a State Transition

The Astrophysical Journal · 2024

We report on an observational campaign on the bright black hole (BH) X-ray binary Swift J1727.8─1613 centered around five observations by the Imaging X-ray Polarimetry Explorer. These observations track for the first time the evolution of the X-ray polarization of a BH X-ray binary across a hard to soft state transition. The 2─8 keV polarization degree decreased from ∼4% to ∼3% across the five observations, but the polarization angle remained oriented in the north─south direction throughout. Based on observations with the Australia Telescope Compact Array, we find that the intrinsic 7.25 GHz radio polarization aligns with the X-ray polarization. Assuming the radio polarization aligns with the jet direction (which can be tested in the future with higher-spatial-resolution images of the jet), our results imply that the X-ray corona is extended in the disk plane, rather than along the jet axis, for the entire hard intermediate state. This in turn implies that the long (≳10 ms) soft lags that we measure with the Neutron star Interior Composition ExploreR are dominated by processes other than pure light-crossing delays. Moreover, we find that the evolution of the soft lag amplitude with spectral state does not follow the trend seen for other sources, implying that Swift J1727.8─1613 is a member of a hitherto undersampled subpopulation.

Constraints on axion-like particles with the Perseus Galaxy Cluster with MAGIC

Physics of the Dark Universe · 2024

Axion-like particles (ALPs) are pseudo-Nambu─Goldstone bosons that emerge in various theories beyond the standard model. These particles can interact with high-energy photons in external magnetic fields, influencing the observed gamma-ray spectrum. This study analyzes 41.3 h of observational data from the Perseus Galaxy Cluster collected with the MAGIC telescopes. We focused on the spectra the radio galaxy in the center of the cluster: NGC 1275. By modeling the magnetic field surrounding this target, we searched for spectral indications of ALP presence. Despite finding no statistical evidence of ALP signatures, we were able to exclude ALP models in the sub-micro electronvolt range. Our analysis improved upon previous work by calculating the full likelihood and statistical coverage for all considered models across the parameter space. Consequently, we achieved the most stringent limits to date for ALP masses around 50 neV, with cross sections down to <mml:math altimg="si1.svg" display="inline" id="d1e3955"><mml:mrow><mml:msub><mml:mrow><mml:mi>g</mml:mi></mml:mrow><mml:mrow><mml:mi>a</mml:mi><mml:mi>γ</mml:mi></mml:mrow></mml:msub><mml:mo linebreak="goodbreak" linebreakstyle="after">=</mml:mo><mml:mn>3</mml:mn><mml:mo linebreak="goodbreak" linebreakstyle="after">×</mml:mo><mml:mn>1</mml:mn><mml:msup><mml:mrow><mml:mn>0</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>12</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math> GeV<SUP>−1</SUP>.

Insights into the broadband emission of the TeV blazar Mrk 501 during the first X-ray polarization measurements

Astronomy and Astrophysics · 2024

Aims. We present the first multiwavelength study of Mrk 501 that contains simultaneous very-high-energy (VHE) γ-ray observations and X-ray polarization measurements from the Imaging X-ray Polarimetry Explorer (IXPE). Methods. We used radio-to-VHE data from a multiwavelength campaign carried out between March 1, 2022, and July 19, 2022 (MJD 59639 to MJD 59779). The observations were performed by MAGIC, Fermi-LAT, NuSTAR, Swift (XRT and UVOT), and several other instruments that cover the optical and radio bands to complement the IXPE pointings. We characterized the dynamics of the broadband emission around the X-ray polarization measurements through its multiband fractional variability and correlations, and compared changes observed in the polarization degree to changes seen in the broadband emission using a multi-zone leptonic scenario. Results. During the IXPE pointings, the VHE state is close to the average behavior, with a 0.2─1 TeV flux of 20%─50% of the emission of the Crab Nebula. Additionally, it shows low variability and a hint of correlation between VHE γ-rays and X-rays. Despite the average VHE activity, an extreme X-ray behavior is measured for the first two IXPE pointings, taken in March 2022 (MJD 59646 to 59648 and MJD 59665 to 59667), with a synchrotron peak frequency &gt; 1 keV. For the third IXPE pointing, in July 2022 (MJD 59769 to 59772), the synchrotron peak shifts toward lower energies and the optical/X-ray polarization degrees drop. All three IXPE epochs show an atypically low Compton dominance in the γ-rays. The X-ray polarization is systematically higher than at lower energies, suggesting an energy stratification of the jet. While during the IXPE epochs the polarization angles in the X-ray, optical, and radio bands align well, we find a clear discrepancy in the optical and radio polarization angles in the middle of the campaign. Such results further support the hypothesis of an energy-stratified jet. We modeled broadband spectra taken simultaneous to the IXPE pointings, assuming a compact zone that dominates in the X-rays and the VHE band, and an extended zone stretching farther downstream in the jet that dominates the emission at lower energies. NuSTAR data allow us to precisely constrain the synchrotron peak and therefore the underlying electron distribution. The change between the different states observed in the three IXPE pointings can be explained by a change in the magnetization and/or the emission region size, which directly connects the shift in the synchrotron peak to lower energies with the drop in the polarization degree. ⋆ The MWL data are available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/685/A117

IXPE observation confirms a high spin in the accreting black hole 4U 1957+115

Astronomy and Astrophysics · 2024

We present the results of the first X-ray polarimetric observation of the low-mass X-ray binary 4U 1957+115, performed with the Imaging X-ray Polarimetry Explorer in May 2023. The binary system has been in a high-soft spectral state since its discovery and is thought to host a black hole. The ∼571 ks observation reveals a linear polarisation degree of 1.9%±0.6% and a polarisation angle of ‒41.°8±7.°9 in the 2-8 keV energy range. Spectral modelling is consistent with the dominant contribution coming from the standard accretion disc, while polarimetric data suggest a significant role of returning radiation: photons that are bent by strong gravity effects and forced to return to the disc surface, where they can be reflected before eventually reaching the observer. In this setting, we find that models with a black hole spin lower than 0.96 and an inclination lower than 50° are disfavoured.

The variability patterns of the TeV blazar PG 1553 + 113 from a decade of MAGIC and multiband observations

Monthly Notices of the Royal Astronomical Society · 2024

PG 1553 + 113 is one of the few blazars with a convincing quasi-periodic emission in the gamma-ray band. The source is also a very high energy (VHE; &gt;100 GeV) gamma-ray emitter. To better understand its properties and identify the underlying physical processes driving its variability, the MAGIC Collaboration initiated a multiyear, multiwavelength monitoring campaign in 2015 involving the OVRO 40-m and Medicina radio telescopes, REM, KVA, and the MAGIC telescopes, Swift and Fermi satellites, and the WEBT network. The analysis presented in this paper uses data until 2017 and focuses on the characterization of the variability. The gamma-ray data show a (hint of a) periodic signal compatible with literature, but the X-ray and VHE gamma-ray data do not show statistical evidence for a periodic signal. In other bands, the data are compatible with the gamma-ray period, but with a relatively high p-value. The complex connection between the low- and high-energy emission and the non-monochromatic modulation and changes in flux suggests that a simple one-zone model is unable to explain all the variability. Instead, a model including a periodic component along with multiple emission zones is required.

Performance and first measurements of the MAGIC stellar intensity interferometer

Monthly Notices of the Royal Astronomical Society · 2024

In recent years, a new generation of optical intensity interferometers has emerged, leveraging the existing infrastructure of Imaging Atmospheric Cherenkov Telescopes (IACTs). The MAGIC telescopes host the MAGIC-SII system (Stellar Intensity Interferometer), implemented to investigate the feasibility and potential of this technique on IACTs. After the first successful measurements in 2019, the system was upgraded and now features a real-time, dead-time-free, 4-channel, GPU-based correlator. These hardware modifications allow seamless transitions between MAGIC's standard very-high-energy gamma-ray observations and optical interferometry measurements within seconds. We establish the feasibility and potential of employing IACTs as competitive optical Intensity Interferometers with minimal hardware adjustments. The measurement of a total of 22 stellar diameters are reported, 9 corresponding to reference stars with previous comparable measurements, and 13 with no prior measurements. A prospective implementation involving telescopes from the forthcoming Cherenkov Telescope Array Observatory's Northern hemisphere array, such as the first prototype of its Large-Sized Telescopes, LST-1, is technically viable. This integration would significantly enhance the sensitivity of the current system and broaden the UV-plane coverage. This advancement would enable the system to achieve competitive sensitivity with the current generation of long-baseline optical interferometers over blue wavelengths.

Discovery of a variable energy-dependent X-ray polarization in the accreting neutron star GX 5‒1

Astronomy and Astrophysics · 2024

We report on the coordinated observations of the neutron star low-mass X-ray binary (NS-LMXB) GX 5‒1 in X-rays (IXPE, NICER, NuSTAR, and INTEGRAL), optical (REM and LCO), near-infrared (REM), mid-infrared (VLT VISIR), and radio (ATCA). This Z-source was observed by IXPE twice in March-April 2023 (Obs. 1 and 2). In the radio band the source was detected, but only upper limits to the linear polarization were obtained at a 3σ level of 6.1% at 5.5 GHz and 5.9% at 9 GHz in Obs. 1 and 12.5% at 5.5 GHz and 20% at 9 GHz in Obs. 2. The mid-IR, near-IR, and optical observations suggest the presence of a compact jet that peaks in the mid- or far-IR. The X-ray polarization degree was found to be 3.7%±0.4% (at 90% confidence level) during Obs. 1 when the source was in the horizontal branch of the Z-track and 1.8%±0.4% during Obs. 2 when the source was in the normal-flaring branch. These results confirm the variation in polarization degree as a function of the position of the source in the color-color diagram, as for previously observed Z-track sources (Cyg X-2 and XTE 1701‒462). Evidence of a variation in the polarization angle of ∼20° with energy is found in both observations, likely related to the different, nonorthogonal polarization angles of the disk and Comptonization components, which peak at different energies.

First characterization of the emission behavior of Mrk 421 from radio to very high-energy gamma rays with simultaneous X-ray polarization measurements

Astronomy and Astrophysics · 2024

<BR /> Aims: We have performed the first broadband study of Mrk 421 from radio to TeV gamma rays with simultaneous measurements of the X-ray polarization from IXPE. <BR /> Methods: The data were collected as part of an extensive multiwavelength campaign carried out between May and June 2022 using MAGIC, Fermi-LAT, NuSTAR, XMM-Newton, Swift, and several optical and radio telescopes to complement IXPE data. <BR /> Results: During the IXPE exposures, the measured 0.2-1 TeV flux was close to the quiescent state and ranged from 25% to 50% of the Crab Nebula without intra-night variability. Throughout the campaign, the very high-energy (VHE) and X-ray emission are positively correlated at a 4σ significance level. The IXPE measurements reveal an X-ray polarization degree that is a factor of 2-5 higher than in the optical/radio bands; that implies an energy-stratified jet in which the VHE photons are emitted co-spatially with the X-rays, in the vicinity of a shock front. The June 2022 observations exhibit a rotation of the X-ray polarization angle. Despite no simultaneous VHE coverage being available during a large fraction of the swing, the Swift-XRT monitoring reveals an X-ray flux increase with a clear spectral hardening. This suggests that flares in high synchrotron peaked blazars can be accompanied by a polarization angle rotation, as observed in some flat spectrum radio quasars. Finally, during the polarization angle rotation, NuSTAR data reveal two contiguous spectral hysteresis loops in opposite directions (clockwise and counterclockwise), implying important changes in the particle acceleration efficiency on approximately hour timescales. <P />All data shown in Figs. 1, 2, 5, 7, and 8 are available at the CDS via anonymous ftp to <A href="https://cdsarc.cds.unistra.fr">cdsarc.cds.unistra.fr</A> (ftp://130.79.128.5) or via <A href="https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/684/A127">https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/684/A127</A>

X-Ray Polarization of the Black Hole X-Ray Binary 4U 1630─47 Challenges the Standard Thin Accretion Disk Scenario

The Astrophysical Journal · 2024

A large energy-dependent X-ray polarization degree is detected by the Imaging X-ray Polarimetry Explorer (IXPE) in the high-soft emission state of the black hole X-ray binary 4U 1630─47. The highly significant detection (at ≈50σ confidence level) of an unexpectedly high polarization, rising from ∼6% at 2 keV to ∼10% at 8 keV, cannot be easily reconciled with standard models of thin accretion disks. In this work, we compare the predictions of different theoretical models with the IXPE data and conclude that the observed polarization properties are compatible with a scenario in which matter accretes onto the black hole through a thin disk covered by a partially ionized atmosphere flowing away at mildly relativistic velocities.

Characterization of High-polarization Stars and Blazars with DIPOL-1 at Sierra Nevada Observatory

The Astronomical Journal · 2024

We report here the performance and first results of the new multiband optical polarimeter DIPOL-1, installed at the Sierra Nevada Observatory 90 cm T90 telescope (SNO, Granada, Spain). DIPOL-1 is equipped with a plane parallel calcite plate and λ/2 retarder for modulating the intensity of two perpendicularly polarized beams, and a high readout speed CMOS camera that allows for fast, time-dense coverage. We characterize the performance of this instrument through a series of tests on zero- and high-polarization standard stars. The instrumental polarization in the Nasmyth focus was well determined, with a very stable contribution of 4.0806% ± 0.0014% in the optical R band. For bright high-polarization standards (m <SUB> R </SUB> &lt; 8) we reach precisions &lt;0.02% in polarization degree and 0.°1 in polarization angle for exposures of 2─4 min. The polarization properties of these stars have been constrained, providing more recent results also about possible variability for future studies of some of the most used calibrators. Moreover, we have tested the capability of observing much fainter objects, in particular through blazar observations, where we reach a precision of &lt;0.5%−0.6% and &lt;0.°5 for faint targets (m <SUB> R </SUB> ∼ 16.5) with exposures of ∼1 hr. For brighter targets (m <SUB> R </SUB> ∼ 14.5−15), we can aim for time-dense observations with errors &lt;0.2%−0.4% and &lt;1°−1.°5 in 5─20 min. We have successfully performed a first campaign with DIPOL-1, detecting significant polarized emission of several blazars, with special attention to the highest ever polarization degree measured from blazar 3C 345 at ∼32%.

Detection of X-Ray Polarization from the Blazar 1ES 1959+650 with the Imaging X-Ray Polarimetry Explorer

The Astrophysical Journal · 2024

Observations of linear polarization in the 2─8 keV energy range with the Imaging X-ray Polarimetry Explorer (IXPE) explore the magnetic field geometry and dynamics of the regions generating nonthermal radiation in relativistic jets of blazars. These jets, particularly in blazars whose spectral energy distribution peaks at X-ray energies, emit X-rays via synchrotron radiation from high-energy particles within the jet. IXPE observations of the X-ray-selected BL Lac─type blazar 1ES 1959+650 on 2022 May 3─4 showed a significant linear polarization degree of Π<SUB>x</SUB> = 8.0% ± 2.3% at an electric-vector position angle ψ <SUB>x</SUB> = 123° ± 8°. However, on 2022 June 9─12, only an upper limit of Π<SUB>x</SUB> ≤ 5.1% could be derived (at the 99% confidence level). The degree of optical polarization at that time, Π<SUB>O</SUB> ∼ 5%, is comparable to the X-ray measurement. We investigate possible scenarios for these findings, including temporal and geometrical depolarization effects. Unlike some other X-ray-selected BL Lac objects, there is no significant chromatic dependence of the measured polarization in 1ES 1959+650, and its low X-ray polarization may be attributed to turbulence in the jet flow with dynamical timescales shorter than 1 day.

Gammapy: Python toolbox for gamma-ray astronomy

Zenodo · 2024

Gammapy analyzes gamma-ray data and creates sky images, spectra and lightcurves, from event lists and instrument response information; it can also determine the position, morphology and spectra of gamma-ray sources. It is used to analyze data from H.E.S.S., Fermi-LAT, HAWC, and the Cherenkov Telescope Array (CTA).

Repeating flaring activity of the blazar AO 0235+164

Astronomy and Astrophysics · 2024

Context. Blazar <ASTROBJ>AO 0235+164</ASTROBJ>, located at a redshift of z = 0.94, has undergone several sharp multi-spectral-range flaring episodes over recent decades. In particular, the episodes that peaked in 2008 and 2015, which were subject to extensive multi-wavelength coverage, exhibited an interesting behavior. <BR /> Aims: We study the actual origin of these two observed flares by constraining the properties of the observed photo-polarimetric variability as well as of the broadband spectral energy distribution and the observed time-evolution behavior of the source. We use ultra-high-resolution total-flux and polarimetric very-long-baseline interferometry (VLBI) imaging. <BR /> Methods: The analysis of VLBI images allowed us to constrain kinematic and geometrical parameters of the 7 mm jet. We used the discrete correlation function to compute the statistical correlation and the delays between emission at different spectral ranges. The multi-epoch modeling of the spectral energy distributions allowed us to propose specific models of the emission; in particular, with the aim to model the unusual spectral features observed in this source in the X-ray region of the spectrum during strong multi spectral-range flares. <BR /> Results: We find that these X-ray spectral features can be explained by an emission component originating in a separate particle distribution than the one responsible for the two standard blazar bumps. This is in agreement with the results of our correlation analysis, where we did not find a strong correlation between the X-ray and the remaining spectral ranges. We find that both external Compton-dominated and synchrotron self-Compton-dominated models are able to explain the observed spectral energy distributions. However, the synchrotron self-Compton models are strongly favored by the delays and geometrical parameters inferred from the observations.

Ordered magnetic fields around the 3C 84 central black hole

Astronomy and Astrophysics · 2024

Context. 3C 84 is a nearby radio source with a complex total intensity structure, showing linear polarisation and spectral patterns. A detailed investigation of the central engine region necessitates the use of very-long-baseline interferometry (VLBI) above the hitherto available maximum frequency of 86 GHz. <BR /> Aims: Using ultrahigh resolution VLBI observations at the currently highest available frequency of 228 GHz, we aim to perform a direct detection of compact structures and understand the physical conditions in the compact region of 3C 84. <BR /> Methods: We used Event Horizon Telescope (EHT) 228 GHz observations and, given the limited (u, v)-coverage, applied geometric model fitting to the data. Furthermore, we employed quasi-simultaneously observed, ancillary multi-frequency VLBI data for the source in order to carry out a comprehensive analysis of the core structure. <BR /> Results: We report the detection of a highly ordered, strong magnetic field around the central, supermassive black hole of 3C 84. The brightness temperature analysis suggests that the system is in equipartition. We also determined a turnover frequency of ν<SUB>m</SUB> = (113 ± 4) GHz, a corresponding synchrotron self-absorbed magnetic field of B<SUB>SSA</SUB> = (2.9 ± 1.6) G, and an equipartition magnetic field of B<SUB>eq</SUB> = (5.2 ± 0.6) G. Three components are resolved with the highest fractional polarisation detected for this object (m<SUB>net</SUB> = (17.0 ± 3.9)%). The positions of the components are compatible with those seen in low-frequency VLBI observations since 2017-2018. We report a steeply negative slope of the spectrum at 228 GHz. We used these findings to test existing models of jet formation, propagation, and Faraday rotation in 3C 84. <BR /> Conclusions: The findings of our investigation into different flow geometries and black hole spins support an advection-dominated accretion flow in a magnetically arrested state around a rapidly rotating supermassive black hole as a model of the jet-launching system in the core of 3C 84. However, systematic uncertainties due to the limited (u, v)-coverage, however, cannot be ignored. Our upcoming work using new EHT data, which offer full imaging capabilities, will shed more light on the compact region of 3C 84.

cta-observatory/cta-lstchain: v0.10.7 - 2024-02-01

Zenodo · 2024

What's Changed Script to compute NSB-tuning parameter for MC at the waveforms level (#1212) @moralejo New script and notebook for the selection of good quality runs (#1190) @moralejo Update docs related to data checks (#1207) @moralejo Set minimum intensity by default to 50 p.e. in example DL3 and IRF configs (#1208) @morcuended Deal with missing modules in modifier.py (#1204) @moralejo Limit scipy to version before 1.12 which introduces a breaking change and unfreeze sphinx version (#1201) @gabemery Contributers @gabemery, @moralejo, @morcuended and @rlopezcoto

Multi-year characterisation of the broad-band emission from the intermittent extreme BL Lac 1ES 2344+514

Astronomy and Astrophysics · 2024

<BR /> Aims: The BL Lac 1ES 2344+514 is known for temporary extreme properties characterised by a shift of the synchrotron spectral energy distribution (SED) peak energy ν<SUB>synch, p</SUB> above 1 keV. While those extreme states have only been observed during high flux levels thus far, additional multi-year observing campaigns are required to achieve a coherent picture. Here, we report the longest investigation of the source from radio to very high energy (VHE) performed so far, focussing on a systematic characterisation of the intermittent extreme states. <BR /> Methods: We organised a monitoring campaign covering a 3-year period from 2019 to 2021. More than ten instruments participated in the observations in order to cover the emission from radio to VHE. In particular, sensitive X-ray measurements by XMM-Newton, NuSTAR, and AstroSat took place simultaneously with multi-hour MAGIC observations, providing an unprecedented constraint of the two SED components for this blazar. <BR /> Results: While our results confirm that 1ES 2344+514 typically exhibits ν<SUB>synch, p</SUB> &gt; 1 keV during elevated flux periods, we also find periods where the extreme state coincides with low flux activity. A strong spectral variability thus happens in the quiescent state, and is likely caused by an increase in the electron acceleration efficiency without a change in the electron injection luminosity. On the other hand, we also report a strong X-ray flare (among the brightest for 1ES 2344+514) without a significant shift of ν<SUB>synch, p</SUB>. During this particular flare, the X-ray spectrum is among the softest of the campaign. It unveils complexity in the spectral evolution, where the common harder-when-brighter trend observed in BL Lacs is violated. By combining Swift-XRT and Swift-UVOT measurements during a low and hard X-ray state, we find an excess of the UV flux with respect to an extrapolation of the X-ray spectrum to lower energies. This UV excess implies that at least two regions significantly contribute to the infrared/optical/ultraviolet/X-ray emission. Using the simultaneous MAGIC, XMM-Newton, NuSTAR, and AstroSat observations, we argue that a region possibly associated with the 10 GHz radio core may explain such an excess. Finally, we investigate a VHE flare, showing an absence of simultaneous variability in the 0.3‒2 keV band. Using time-dependent leptonic modelling, we show that this behaviour, in contradiction to single-zone scenarios, can instead be explained by a two-component model.

MAGIC detection of GRB 201216C at z = 1.1

Monthly Notices of the Royal Astronomical Society · 2024

Gamma-ray bursts (GRBs) are explosive transient events occurring at cosmological distances, releasing a large amount of energy as electromagnetic radiation over several energy bands. We report the detection of the long GRB 201216C by the MAGIC telescopes. The source is located at z = 1.1 and thus it is the farthest one detected at very high energies. The emission above 70 GeV of GRB 201216C is modelled together with multiwavelength data within a synchrotron and synchrotron self-Compton (SSC) scenario. We find that SSC can explain the broad-band data well from the optical to the very-high-energy band. For the late-time radio data, a different component is needed to account for the observed emission. Differently from previous GRBs detected in the very-high-energy range, the model for GRB 201216C strongly favours a wind-like medium. The model parameters have values similar to those found in past studies of the afterglows of GRBs detected up to GeV energies.

Magnetic field properties inside the jet of Mrk 421. Multiwavelength polarimetry, including the Imaging X-ray Polarimetry Explorer

Astronomy and Astrophysics · 2024

<BR /> Aims: We aim to probe the magnetic field geometry and particle acceleration mechanism in the relativistic jets of supermassive black holes. <BR /> Methods: We conducted a polarimetry campaign from radio to X-ray wavelengths of the high-synchrotron-peak (HSP) blazar Mrk 421, including Imaging X-ray Polarimetry Explorer (IXPE) measurements from 2022 December 6-8. During the IXPE observation, we also monitored Mrk 421 using Swift-XRT and obtained a single observation with XMM-Newton to improve the X-ray spectral analysis. The time-averaged X-ray polarization was determined consistently using the event-by-event Stokes parameter analysis, spectropolarimetric fit, and maximum likelihood methods. We examined the polarization variability over both time and energy, the former via analysis of IXPE data obtained over a time span of 7 months. <BR /> Results: We detected X-ray polarization of Mrk 421 with a degree of Π<SUB>X</SUB> = 14 ± 1% and an electric-vector position angle ψ<SUB>X</SUB> = 107 ± 3° in the 2-8 keV band. From the time variability analysis, we find a significant episodic variation in ψ<SUB>X</SUB>. During the 7 months from the first IXPE pointing of Mrk 421 in 2022 May, ψ<SUB>X</SUB> varied in the range 0° to 180°, while Π<SUB>X</SUB> remained relatively constant within ∼10-15%. Furthermore, a swing in ψ<SUB>X</SUB> in 2022 June was accompanied by simultaneous spectral variations. The results of the multiwavelength polarimetry show that Π<SUB>X</SUB> was generally ∼2-3 times greater than Π at longer wavelengths, while ψ fluctuated. Additionally, based on radio, infrared, and optical polarimetry, we find that the rotation of ψ occurred in the opposite direction with respect to the rotation of ψ<SUB>X</SUB> and over longer timescales at similar epochs. <BR /> Conclusions: The polarization behavior observed across multiple wavelengths is consistent with previous IXPE findings for HSP blazars. This result favors the energy-stratified shock model developed to explain variable emission in relativistic jets. We considered two versions of the model, one with linear and the other with radial stratification geometry, to explain the rotation of ψ<SUB>X</SUB>. The accompanying spectral variation during the ψ<SUB>X</SUB> rotation can be explained by a fluctuation in the physical conditions, for example in the energy distribution of relativistic electrons. The opposite rotation direction of ψ between the X-ray and longer wavelength polarization accentuates the conclusion that the X-ray emitting region is spatially separated from that at longer wavelengths. Moreover, we identify a highly polarized knot of radio emission moving down the parsec-scale jet during the episode of ψ<SUB>X</SUB> rotation, although it is unclear whether there is any connection between the two events.

Highly Significant Detection of X-Ray Polarization from the Brightest Accreting Neutron Star Sco X-1

The Astrophysical Journal · 2024

The Imaging X-ray Polarimetry Explorer measured with high significance the X-ray polarization of the brightest Z-source, Sco X-1, resulting in the nominal 2-8 keV energy band in a polarization degree of 1.0% ± 0.2% and a polarization angle of 8° ± 6° at a 90% confidence level. This observation was strictly simultaneous with observations performed by NICER, NuSTAR, and Insight-HXMT, which allowed for a precise characterization of its broadband spectrum from soft to hard X-rays. The source has been observed mainly in its soft state, with short periods of flaring. We also observed low-frequency quasiperiodic oscillations. From a spectropolarimetric analysis, we associate a polarization to the accretion disk at &lt;3.2% at 90% confidence level, compatible with expectations for an electron scattering dominated optically thick atmosphere at the Sco X-1 inclination of ~44°; for the higher-energy Comptonized component, we obtain a polarization of 1.3% ± 0.4%, in agreement with expectations for a slab of Thomson optical depth of ~7 and an electron temperature of ~3 keV. A polarization rotation with respect to previous observations by OSO-8 and PolarLight, and also with respect to the radio-jet position angle, is observed. This result may indicate a variation of the polarization with the source state that can be related to relativistic precession or a change in the corona geometry with the accretion flow.

First X-Ray Polarization Measurement Confirms the Low Black Hole Spin in LMC X-3

The Astrophysical Journal · 2024

X-ray polarization is a powerful tool to investigate the geometry of accreting material around black holes, allowing independent measurements of the black hole spin and orientation of the innermost parts of the accretion disk. We perform X-ray spectropolarimetric analysis of an X-ray binary system in the Large Magellanic Cloud, LMC X-3, that hosts a stellar-mass black hole, known to be persistently accreting since its discovery. We report the first detection of the X-ray polarization in LMC X-3 with the Imaging X-ray Polarimetry Explorer, and find the average polarization degree (PD) of 3.2% ± 0.6% and a constant polarization angle of -42° ± 6° over the 2-8 keV range. Using accompanying spectroscopic observations by NICER, NuSTAR, and the Neil Gehrels Swift observatories, we confirm previous measurements of the black hole spin via the X-ray continuum method, a ≈ 0.2. From polarization analysis only, we found consistent results with low black hole spin, with an upper limit of a &lt; 0.7 at a 90% confidence level. A slight increase in the PD with energy, similar to other black hole X-ray binaries in the soft state, is suggested from the data but with a low statistical significance.

cta-observatory/cta-lstchain: v0.10.5 - 2023-12-01

Zenodo · 2023

What's Changed Add missing wild card in pedestal search function to allow looking for symlinks in glob (#1185) @FrancaCassol Estimate cleaning threshold with CatB pedestals if availables (#1143) @FrancaCassol Notebook for sensitivity calculation (#1158) @moralejo Update interpolation code to pyirf v0.10 API (#1184) @morcuended Remove unused get_cleaning_parameters function (#1170) @aaguasca Add possibility to copy and not merge some keys by lstchain_merge_hdf5_files (#1174) @FrancaCassol Add Cat-B calibration onsite scripts (#1147) @FrancaCassol Make run_summary script work with ctar1 data, store format string (#1179) @maxnoe Create script to merge run summaries with drive logs into a single file (#880) @maxnoe Get run type from TCU if it's available (#1173) @marialainez add default compression to write_dataframe function to compress dl2 (#1165) @vuillaut Change master to main to trigger docs deployment (#1169) @morcuended request ctaplot 0.6.4 to fix the seaborn style issue (#1167) @vuillaut Update links to ctapipe docs in README (#1162) @morcuended Pin matplotlib to 3.7.0 to avoid setting deprecated seaborn styles in matplotlib 3.8 (#1166) @morcuended Write modified images from dl1ab in output DL1 files and fix add_config_metadata (#1145) @vuillaut Updated run_type in summary test (#1156) @morcuended Fix treatment of Nan values in filter scan fit (#1153) @FrancaCassol Contributers @FrancaCassol, @aaguasca, @marialainez, @maxnoe, @moralejo, @morcuended, @rlopezcoto and @vuillaut

Performance of the joint LST-1 and MAGIC observations evaluated with Crab Nebula data

Astronomy and Astrophysics · 2023

<BR /> Aims: Large-Sized Telescope 1 (LST-1), the prototype for the Large-Sized Telescope at the upcoming Cherenkov Telescope Array Observatory, is concluding its commissioning phase at the Observatorio del Roque de los Muchachos on the island of La Palma. The proximity of LST-1 to the two MAGIC (Major Atmospheric Gamma Imaging Cherenkov) telescopes makes it possible to carry out observations of the same gamma-ray events with both systems. <BR /> Methods: We describe the joint LST-1+MAGIC analysis pipeline and used simultaneous Crab Nebula observations and Monte Carlo simulations to assess the performance of the three-telescope system. The addition of the LST-1 telescope allows for the recovery of events in which one of the MAGIC images is too dim to survive analysis quality cuts. <BR /> Results: Thanks to the resulting increase in the collection area and stronger background rejection, we found a significant improvement in sensitivity, allowing for the detection of 30% weaker fluxes in the energy range between 200 GeV and 3 TeV. The spectrum of the Crab Nebula, reconstructed in the energy range between ~60 GeV and ~10 TeV, is in agreement with previous measurements.

Discovery of X-Ray Polarization from the Black Hole Transient Swift J1727.8-1613

The Astrophysical Journal · 2023

We report the first detection of the X-ray polarization of the bright transient Swift J1727.8-1613 with the Imaging X-ray Polarimetry Explorer. The observation was performed at the beginning of the 2023 discovery outburst, when the source resided in the bright hard state. We find a time- and energy-averaged polarization degree of 4.1% ± 0.2% and a polarization angle of 2.°2 ± 1.°3 (errors at 68% confidence level; this translates to ~20σ significance of the polarization detection). This finding suggests that the hot corona emitting the bulk of the detected X-rays is elongated, rather than spherical. The X-ray polarization angle is consistent with that found in submillimeter wavelengths. Since the submillimeter polarization was found to be aligned with the jet direction in other X-ray binaries, this indicates that the corona is elongated orthogonal to the jet.

Star tracking for pointing determination of Imaging Atmospheric Cherenkov Telescopes. Application to the Large-Sized Telescope of the Cherenkov Telescope Array

Astronomy and Astrophysics · 2023

We present a novel approach to the determination of the pointing of Imaging Atmospheric Cherenkov Telescopes (IACTs) using the trajectories of the stars in their camera's field of view. The method starts with the reconstruction of the star positions from the Cherenkov camera data, taking into account the point spread function of the telescope, to achieve a satisfying reconstruction accuracy of the pointing position. A simultaneous fit of all reconstructed star trajectories is then performed with the orthogonal distance regression (ODR) method. ODR allows us to correctly include the star position uncertainties and use the time as an independent variable. Having the time as an independent variable in the fit makes it better suitable for various star trajectories. This method can be applied to any IACT and requires neither specific hardware nor interface or special data-taking mode. In this paper, we use the Large-Sized Telescope (LST) data to validate it as a useful tool to improve the determination of the pointing direction during regular data taking. The simulation studies show that the accuracy and precision of the method are comparable with the design requirements on the pointing accuracy of the LST (≤14″). With the typical LST event acquisition rate of 10 kHz, the method can achieve up to 50 Hz pointing monitoring rate, compared to 𝒪(1) Hz achievable with standard techniques. The application of the method to the LST prototype (LST-1) commissioning data shows the stable pointing performance of the telescope.

The X-ray polarization of the Seyfert 1 galaxy IC 4329A

Monthly Notices of the Royal Astronomical Society · 2023

We present an X-ray spectro-polarimetric analysis of the bright Seyfert galaxy IC 4329A. The Imaging X-ray Polarimetry Explorer (IXPE) observed the source for ~500 ks, supported by XMM-Newton (~60 ks) and NuSTAR (~80 ks) exposures. We detect polarization in the 2-8 keV band with 2.97σ confidence. We report a polarization degree of 3.3 ± 1.1 per cent and a polarization angle of 78° ± 10° (errors are 1σ confidence). The X-ray polarization is consistent with being aligned with the radio jet, albeit partially due to large uncertainties on the radio position angle. We jointly fit the spectra from the three observatories to constrain the presence of a relativistic reflection component. From this, we obtain constraints on the inclination angle to the inner disc (&lt;39° at 99 per cent confidence) and the disc inner radius (&lt;11 gravitational radii at 99 per cent confidence), although we note that modelling systematics in practice add to the quoted statistical error. Our spectropolarimetric modelling indicates that the 2-8 keV polarization is consistent with being dominated by emission directly observed from the X-ray corona, but the polarization of the reflection component is completely unconstrained. Our constraints on viewer inclination and polarization degree tentatively favour more asymmetric, possibly out-flowing, coronal geometries that produce more highly polarized emission, but the coronal geometry is unconstrained at the 3σ level.

Observations of the Crab Nebula and Pulsar with the Large-sized Telescope Prototype of the Cherenkov Telescope Array

The Astrophysical Journal · 2023

The Cherenkov Telescope Array (CTA) is a next-generation ground-based observatory for gamma-ray astronomy at very high energies. The Large-Sized Telescope prototype (LST-1) is located at the CTA-North site, on the Canary Island of La Palma. LSTs are designed to provide optimal performance in the lowest part of the energy range covered by CTA, down to ≃20 GeV. LST-1 started performing astronomical observations in 2019 November, during its commissioning phase, and it has been taking data ever since. We present the first LST-1 observations of the Crab Nebula, the standard candle of very-high-energy gamma-ray astronomy, and use them, together with simulations, to assess the performance of the telescope. LST-1 has reached the expected performance during its commissioning period-only a minor adjustment of the preexisting simulations was needed to match the telescope's behavior. The energy threshold at trigger level is around 20 GeV, rising to ≃30 GeV after data analysis. Performance parameters depend strongly on energy, and on the strength of the gamma-ray selection cuts in the analysis: angular resolution ranges from 0.°12-0.°40, and energy resolution from 15%-50%. Flux sensitivity is around 1.1% of the Crab Nebula flux above 250 GeV for a 50 hr observation (12% for 30 minutes). The spectral energy distribution (in the 0.03-30 TeV range) and the light curve obtained for the Crab Nebula agree with previous measurements, considering statistical and systematic uncertainties. A clear periodic signal is also detected from the pulsar at the center of the Nebula.

Gammapy: A Python package for gamma-ray astronomy

Astronomy and Astrophysics · 2023

Context. Traditionally, TeV-γ-ray astronomy has been conducted by experiments employing proprietary data and analysis software. However, the next generation of γ-ray instruments, such as the Cherenkov Telescope Array Observatory (CTAO), will be operated as open observatories. Alongside the data, they will also make the associated software tools available to a wider community. This necessity prompted the development of open, high-level, astronomical software customized for high-energy astrophysics. <BR /> Aims: In this article, we present Gammapy, an open-source Python package for the analysis of astronomical γ-ray data, and illustrate the functionalities of its first long-term-support release, version 1.0. Built on the modern Python scientific ecosystem, Gammapy provides a uniform platform for reducing and modeling data from different γ-ray instruments for many analysis scenarios. Gammapy complies with several well-established data conventions in high-energy astrophysics, providing serialized data products that are interoperable with other software packages. <BR /> Methods: Starting from event lists and instrument response functions, Gammapy provides functionalities to reduce these data by binning them in energy and sky coordinates. Several techniques for background estimation are implemented in the package to handle the residual hadronic background affecting γ-ray instruments. After the data are binned, the flux and morphology of one or more γ-ray sources can be estimated using Poisson maximum likelihood fitting and assuming a variety of spectral, temporal, and spatial models. Estimation of flux points, likelihood profiles, and light curves is also supported. <BR /> Results: After describing the structure of the package, we show, using publicly available gamma-ray data, the capabilities of Gammapy in multiple traditional and novel γ-ray analysis scenarios, such as spectral and spectro-morphological modeling and estimations of a spectral energy distribution and a light curve. Its flexibility and its power are displayed in a final multi-instrument example, where datasets from different instruments, at different stages of data reduction, are simultaneously fitted with an astrophysical flux model.

Discovery of X-ray polarization angle rotation in the jet from blazar Mrk 421.

Nature Astronomy · 2023

The magnetic-field conditions in astrophysical relativistic jets can be probed by multiwavelength polarimetry, which has been recently extended to X-rays. For example, one can track how the magnetic field changes in the flow of the radiating particles by observing rotations of the electric vector position angle Ψ. Here we report the discovery of a Ψ<SUB>X</SUB> rotation in the X-ray band in the blazar Markarian 421 at an average flux state. Across the 5 days of Imaging X-ray Polarimetry Explorer observations on 4-6 and 7-9 June 2022, Ψ<SUB>X</SUB> rotated in total by ≥360°. Over the two respective date ranges, we find constant, within uncertainties, rotation rates (80 ± 9° per day and 91 ± 8° per day) and polarization degrees (Π<SUB>X</SUB> = 10% ± 1%). Simulations of a random walk of the polarization vector indicate that it is unlikely that such rotation(s) are produced by a stochastic process. The X-ray-emitting site does not completely overlap the radio, infrared and optical emission sites, as no similar rotation of Ψ was observed in quasi-simultaneous data at longer wavelengths. We propose that the observed rotation was caused by a helical magnetic structure in the jet, illuminated in the X-rays by a localized shock propagating along this helix. The optically emitting region probably lies in a sheath surrounding an inner spine where the X-ray radiation is released.

cta-observatory/cta-lstchain: v0.9.14 – 2023-09-25

Zenodo · 2023

What's Changed This release includes a hotfix for version 0.9.x for the wrong EDISP normalization in pyirf. To fix existing DL3 files, you can use the script attached to the pyirf 0.10 release here: https://github.com/cta-observatory/pyirf/releases/download/v0.10.0/fix_edisp.py Properly normalize edisp in lstchain v0.9 (#1164) @morcuended Contributors @morcuended and @maxnoe

Uncovering the geometry of the hot X-ray corona in the Seyfert galaxy NGC 4151 with IXPE

Monthly Notices of the Royal Astronomical Society · 2023

We present an X-ray spectropolarimetric analysis of the bright Seyfert galaxy NGC 4151. The source has been observed with the Imaging X-ray Polarimetry Explorer (IXPE) for 700 ks, complemented with simultaneous XMM-Newton (50 ks) and NuSTAR (100 ks) pointings. A polarization degree Π = 4.9 ± 1.1 per cent and angle Ψ = 86° ± 7° east of north (68 per cent confidence level) are measured in the 2-8 keV energy range. The spectropolarimetric analysis shows that the polarization could be entirely due to reflection. Given the low reflection flux in the IXPE band, this requires, however, a reflection with a very large (&gt;38 per cent) polarization degree. Assuming more reasonable values, a polarization degree of the hot corona ranging from ~4 to ~8 per cent is found. The observed polarization degree excludes a 'spherical' lamppost geometry for the corona, suggesting instead a slab-like geometry, possibly a wedge, as determined via Monte Carlo simulations. This is further confirmed by the X-ray polarization angle, which coincides with the direction of the extended radio emission in this source, supposed to match the disc axis. NGC 4151 is the first active galactic nucleus with an X-ray polarization measure for the corona, illustrating the capabilities of X-ray polarimetry and IXPE in unveiling its geometry.

Sensitivity of the Cherenkov Telescope Array to TeV photon emission from the Large Magellanic Cloud

Monthly Notices of the Royal Astronomical Society · 2023

A deep survey of the Large Magellanic Cloud at ~0.1-100 TeV photon energies with the Cherenkov Telescope Array is planned. We assess the detection prospects based on a model for the emission of the galaxy, comprising the four known TeV emitters, mock populations of sources, and interstellar emission on galactic scales. We also assess the detectability of 30 Doradus and SN 1987A, and the constraints that can be derived on the nature of dark matter. The survey will allow for fine spectral studies of N 157B, N 132D, LMC P3, and 30 Doradus C, and half a dozen other sources should be revealed, mainly pulsar-powered objects. The remnant from SN 1987A could be detected if it produces cosmic-ray nuclei with a flat power-law spectrum at high energies, or with a steeper index 2.3-2.4 pending a flux increase by a factor of &gt;3-4 over ~2015-2035. Large-scale interstellar emission remains mostly out of reach of the survey if its &gt;10 GeV spectrum has a soft photon index ~2.7, but degree-scale 0.1-10 TeV pion-decay emission could be detected if the cosmic-ray spectrum hardens above &gt;100 GeV. The 30 Doradus star-forming region is detectable if acceleration efficiency is on the order of 1-10 per cent of the mechanical luminosity and diffusion is suppressed by two orders of magnitude within &lt;100 pc. Finally, the survey could probe the canonical velocity-averaged cross-section for self-annihilation of weakly interacting massive particles for cuspy Navarro-Frenk-White profiles.

Sensitivity of the Cherenkov Telescope Array to spectral signatures of hadronic PeVatrons with application to Galactic Supernova Remnants

Astroparticle Physics · 2023

The local Cosmic Ray (CR) energy spectrum exhibits a spectral softening at energies around 3 PeV. Sources which are capable of accelerating hadrons to such energies are called hadronic PeVatrons. However, hadronic PeVatrons have not yet been firmly identified within the Galaxy. Several source classes, including Galactic Supernova Remnants (SNRs), have been proposed as PeVatron candidates. The potential to search for hadronic PeVatrons with the Cherenkov Telescope Array (CTA) is assessed. The focus is on the usage of very high energy γ-ray spectral signatures for the identification of PeVatrons. Assuming that SNRs can accelerate CRs up to knee energies, the number of Galactic SNRs which can be identified as PeVatrons with CTA is estimated within a model for the evolution of SNRs. Additionally, the potential of a follow-up observation strategy under moonlight conditions for PeVatron searches is investigated. Statistical methods for the identification of PeVatrons are introduced, and realistic Monte-Carlo simulations of the response of the CTA observatory to the emission spectra from hadronic PeVatrons are performed. Based on simulations of a simplified model for the evolution for SNRs, the detection of a γ-ray signal from in average 9 Galactic PeVatron SNRs is expected to result from the scan of the Galactic plane with CTA after 10 h of exposure. CTA is also shown to have excellent potential to confirm these sources as PeVatrons in deep observations with O(100) hours of exposure per source.

Observations of the Crab Nebula and Pulsar with the Large-Sized Telescope prototype of the Cherenkov Telescope Array Data Points

Zenodo · 2023

This repository includes code and data to reproduce the figures (only the data points, not the entire data analysis) of the paper entitled Observations of the Crab Nebula and Pulsar with the Large-Sized Telescope prototype of the Cherenkov Telescope Array for the CTA LST collaboration.

X-ray polarization evidence for a 200-year-old flare of Sgr A<SUP>*</SUP>

Nature · 2023

The centre of the Milky Way Galaxy hosts a black hole with a solar mass of about 4 million (Sagittarius A<SUP>*</SUP> (Sgr A)) that is very quiescent at present with a luminosity many orders of magnitude below those of active galactic nuclei<SUP>1</SUP>. Reflection of X-rays from Sgr A<SUP>*</SUP> by dense gas in the Galactic Centre region offers a means to study its past flaring activity on timescales of hundreds and thousands of years<SUP>2</SUP>. The shape of the X-ray continuum and the strong fluorescent iron line observed from giant molecular clouds in the vicinity of Sgr A<SUP>*</SUP> are consistent with the reflection scenario<SUP>3-5</SUP>. If this interpretation is correct, the reflected continuum emission should be polarized<SUP>6</SUP>. Here we report observations of polarized X-ray emission in the direction of the molecular clouds in the Galactic Centre using the Imaging X-ray Polarimetry Explorer. We measure a polarization degree of 31% ± 11%, and a polarization angle of ‒48° ± 11°. The polarization angle is consistent with Sgr A<SUP>*</SUP> being the primary source of the emission, and the polarization degree implies that some 200 years ago, the X-ray luminosity of Sgr A<SUP>*</SUP> was briefly comparable to that of a Seyfert galaxy.

cta-observatory/ctapipe: v0.19.3

Zenodo · 2023

Summary This is a bugfix release, find the list of merged pull requests below, the changelog is available on readthedocs. Contributors @LukasBeiske, @clara-escanuela, @kosack and @maxnoe What has changed since v0.19.2 Pull-requests containing changes of multiple nature are repeated. Render changelog for 0.19.3 (#2362) @maxnoe Improve docstring and validation of parameters in CameraGeometry (#2361) @maxnoe Fix removal of telescope events in software trigger, fixes #2356 (#2357) @maxnoe Add support for using toymodel in telescope frame (#2349) @maxnoe Fix pixel picking for multiple displays in same figure (#2358) @maxnoe Fill reference_location into SubarrayDescription.tel_coords (#2354) @maxnoe fix help string in disp reconstructor (#2352) @maxnoe Add support for hillas parameters in TelescopeFrame to CameraDisplay (#2347) @maxnoe Fix typo in merge tool flag definition (#2341) @LukasBeiske Fix return value of NullDataVolumeReducer (#2340) @maxnoe Peak time units conversion for the FlashCam reconstructor (#2337) @clara-escanuela Maintenance Improve error messages of expression engine (#2350) @maxnoe

Multimessenger Characterization of Markarian 501 during Historically Low X-Ray and γ-Ray Activity

The Astrophysical Journal Supplement Series · 2023

We study the broadband emission of Mrk 501 using multiwavelength observations from 2017 to 2020 performed with a multitude of instruments, involving, among others, MAGIC, Fermi's Large Area Telescope (LAT), NuSTAR, Swift, GASP-WEBT, and the Owens Valley Radio Observatory. Mrk 501 showed an extremely low broadband activity, which may help to unravel its baseline emission. Nonetheless, significant flux variations are detected at all wave bands, with the highest occurring at X-rays and very-high-energy (VHE) γ-rays. A significant correlation (&gt;3σ) between X-rays and VHE γ-rays is measured, supporting leptonic scenarios to explain the variable parts of the emission, also during low activity. This is further supported when we extend our data from 2008 to 2020, and identify, for the first time, significant correlations between the Swift X-Ray Telescope and Fermi-LAT. We additionally find correlations between high-energy γ-rays and radio, with the radio lagging by more than 100 days, placing the γ-ray emission zone upstream of the radio-bright regions in the jet. Furthermore, Mrk 501 showed a historically low activity in X-rays and VHE γ-rays from mid-2017 to mid-2019 with a stable VHE flux (&gt;0.2 TeV) of 5% the emission of the Crab Nebula. The broadband spectral energy distribution (SED) of this 2 yr long low state, the potential baseline emission of Mrk 501, can be characterized with one-zone leptonic models, and with (lepto)-hadronic models fulfilling neutrino flux constraints from IceCube. We explore the time evolution of the SED toward the low state, revealing that the stable baseline emission may be ascribed to a standing shock, and the variable emission to an additional expanding or traveling shock.

Simultaneous space and phase resolved X-ray polarimetry of the Crab pulsar and nebula

Nature Astronomy · 2023

The Crab pulsar and its nebula are among the most studied astrophysical systems, and constitute one of the most promising environments where high-energy processes and particle acceleration can be investigated. They are the only objects for which significant X-ray polarization was detected in the past. Here we present the Imaging X-ray Polarimetry Explorer (IXPE) observation of the Crab pulsar and nebula. The total pulsar pulsed emission in the [2-8] keV energy range is unpolarized. Significant polarization up to 15% is detected in the core of the main peak. The nebula has a total space integrated polarized degree of 20% and polarization angle of 145°. The polarized maps show a large variation in the local polarization, and regions with a polarized degree up to 45-50%. The polarization pattern suggests a predominantly toroidal magnetic field. Our findings for the pulsar are inconsistent with most inner magnetospheric models, and suggest emission is more likely to come from the wind region. For the nebula, the polarization map suggests a patchy distribution of turbulence, uncorrelated with the intensity, in contrast with simple expectations from numerical models.

Multiwavelength study of the galactic PeVatron candidate LHAASO J2108+5157

Astronomy and Astrophysics · 2023

Context. Several new ultrahigh-energy (UHE) γ-ray sources have recently been discovered by the Large High Altitude Air Shower Observatory (LHAASO) collaboration. These represent a step forward in the search for the so-called Galactic PeVatrons, the enigmatic sources of the Galactic cosmic rays up to PeV energies. However, it has been shown that multi-TeV γ-ray emission does not necessarily prove the existence of a hadronic accelerator in the source; indeed this emission could also be explained as inverse Compton scattering from electrons in a radiation-dominated environment. A clear distinction between the two major emission mechanisms would only be made possible by taking into account multi-wavelength data and detailed morphology of the source. <BR /> Aims: We aim to understand the nature of the unidentified source LHAASO J2108+5157, which is one of the few known UHE sources with no very high-energy (VHE) counterpart. <BR /> Methods: We observed LHAASO J2108+5157 in the X-ray band with XMM-Newton in 2021 for a total of 3.8 hours and at TeV energies with the Large-Sized Telescope prototype (LST-1), yielding 49 hours of good-quality data. In addition, we analyzed 12 years of Fermi-LAT data, to better constrain emission of its high-energy (HE) counterpart 4FGL J2108.0+5155. We used naima and jetset software packages to examine the leptonic and hadronic scenario of the multi-wavelength emission of the source. <BR /> Results: We found an excess (3.7σ) in the LST-1 data at energies E &gt; 3 TeV. Further analysis of the whole LST-1 energy range, assuming a point-like source, resulted in a hint (2.2σ) of hard emission, which can be described with a single power law with a photon index of Γ = 1.6 ± 0.2 the range of 0.3 ‒ 100 TeV. We did not find any significant extended emission that could be related to a supernova remnant (SNR) or pulsar wind nebula (PWN) in the XMM-Newton data, which puts strong constraints on possible synchrotron emission of relativistic electrons. We revealed a new potential hard source in Fermi-LAT data with a significance of 4σ and a photon index of Γ = 1.9 ± 0.2, which is not spatially correlated with LHAASO J2108+5157, but including it in the source model we were able to improve spectral representation of the HE counterpart 4FGL J2108.0+5155. <BR /> Conclusions: The LST-1 and LHAASO observations can be explained as inverse Compton-dominated leptonic emission of relativistic electrons with a cutoff energy of 100<SUB>‒30</SUB><SUP>+70</SUP> TeV. The low magnetic field in the source imposed by the X-ray upper limits on synchrotron emission is compatible with a hypothesis of a PWN or a TeV halo. Furthermore, the spectral properties of the HE counterpart are consistent with a Geminga-like pulsar, which would be able to power the VHE-UHE emission. Nevertheless, the lack of a pulsar in the neighborhood of the UHE source is a challenge to the PWN/TeV-halo scenario. The UHE γ rays can also be explained as π<SUP>0</SUP> decay-dominated hadronic emission due to interaction of relativistic protons with one of the two known molecular clouds in the direction of the source. Indeed, the hard spectrum in the LST-1 band is compatible with protons escaping a shock around a middle-aged SNR because of their high low-energy cut-off, but the origin of the HE γ-ray emission remains an open question.

Gammapy: Python toolbox for gamma-ray astronomy

Zenodo · 2023

Gammapy analyzes gamma-ray data and creates sky images, spectra and lightcurves, from event lists and instrument response information; it can also determine the position, morphology and spectra of gamma-ray sources. It is used to analyze data from H.E.S.S., Fermi-LAT, HAWC, and the Cherenkov Telescope Array (CTA).

Accretion geometry of the neutron star low mass X-ray binary Cyg X-2 from X-ray polarization measurements

Monthly Notices of the Royal Astronomical Society · 2023

We report spectro-polarimetric results of an observational campaign of the bright neutron star low-mass X-ray binary Cyg X-2 simultaneously observed by IXPE, NICER, and INTEGRAL. Consistently with previous results, the broad-band spectrum is characterized by a lower-energy component, attributed to the accretion disc with kT<SUB>in</SUB> ≈ 1 keV, plus unsaturated Comptonization in thermal plasma with temperature kT<SUB>e</SUB> = 3 keV and optical depth τ ≈ 4, assuming a slab geometry. We measure the polarization degree in the 2-8 keV band P = 1.8 ± 0.3 per cent and polarization angle ϕ = 140° ± 4°, consistent with the previous X-ray polarimetric measurements by OSO-8 as well as with the direction of the radio jet which was earlier observed from the source. While polarization of the disc spectral component is poorly constrained with the IXPE data, the Comptonized emission has a polarization degree P = 4.0 ± 0.7 per cent and a polarization angle aligned with the radio jet. Our results strongly favour a spreading layer at the neutron star surface as the main source of the polarization signal. However, we cannot exclude a significant contribution from reflection off the accretion disc, as indicated by the presence of the iron fluorescence line.

MAGIC observations provide compelling evidence of hadronic multi-TeV emission from the putative PeVatron SNR G106.3+2.7

Astronomy and Astrophysics · 2023

Context. Certain types of supernova remnants (SNRs) in our Galaxy are assumed to be PeVatrons, capable of accelerating cosmic rays (CRs) to ~ PeV energies. However, conclusive observational evidence for this has not yet been found. The SNR G106.3+2.7, detected at 1-100 TeV energies by different γ-ray facilities, is one of the most promising PeVatron candidates. This SNR has a cometary shape, which can be divided into a head and a tail region with different physical conditions. However, in which region the 100 TeV emission is produced has not yet been identified because of the limited position accuracy and/or angular resolution of existing observational data. Additionally, it remains unclear as to whether the origin of the γ-ray emission is leptonic or hadronic. <BR /> Aims: With the better angular resolution provided by new MAGIC data compared to earlier γ-ray datasets, we aim to reveal the acceleration site of PeV particles and the emission mechanism by resolving the SNR G106.3+2.7 with 0.1° resolution at TeV energies. <BR /> Methods: We observed the SNR G106.3+2.7 using the MAGIC telescopes for 121.7 h in total - after quality cuts - between May 2017 and August 2019. The analysis energy threshold is ~0.2 TeV, and the angular resolution is 0.07‒0.1°. We examined the γ-ray spectra of different parts of the emission, whilst benefitting from the unprecedented statistics and angular resolution at these energies provided by our new data. We also used measurements at other wavelengths such as radio, X-rays, GeV γ-rays, and 10 TeV γ-rays to model the emission mechanism precisely. <BR /> Results: We detect extended γ-ray emission spatially coincident with the radio continuum emission at the head and tail of SNR G106.3+2.7. The fact that we detect a significant γ-ray emission with energies above 6.0 TeV from only the tail region suggests that the emissions above 10 TeV detected with air shower experiments (Milagro, HAWC, Tibet ASγ and LHAASO) are emitted only from the SNR tail. Under this assumption, the multi-wavelength spectrum of the head region can be explained with either hadronic or leptonic models, while the leptonic model for the tail region is in contradiction with the emission above 10 TeV and X-rays. In contrast, the hadronic model could reproduce the observed spectrum at the tail by assuming a proton spectrum with a cutoff energy of ~1 PeV for that region. Such high-energy emission in this middle-aged SNR (4‒10 kyr) can be explained by considering a scenario where protons escaping from the SNR in the past interact with surrounding dense gases at present. <BR /> Conclusions: The γ-ray emission region detected with the MAGIC telescopes in the SNR G106.3+2.7 is extended and spatially coincident with the radio continuum morphology. The multi-wavelength spectrum of the emission from the tail region suggests proton acceleration up to ~PeV, while the emission mechanism of the head region could either be hadronic or leptonic.

Long-term multi-wavelength study of 1ES 0647+250

Astronomy and Astrophysics · 2023

Context. The BL Lac object 1ES 0647+250 is one of the few distant γ-ray emitting blazars detected at very high energies (VHEs; ≳100 GeV) during a non-flaring state. It was detected with the MAGIC telescopes during a period of low activity in the years 2009‒2011 as well as during three flaring activities in the years 2014, 2019, and 2020, with the highest VHE flux in the last epoch. An extensive multi-instrument data set was collected as part of several coordinated observing campaigns over these years. <BR /> Aims: We aim to characterise the long-term multi-band flux variability of 1ES 0647+250, as well as its broadband spectral energy distribution (SED) during four distinct activity states selected in four different epochs, in order to constrain the physical parameters of the blazar emission region under certain assumptions. <BR /> Methods: We evaluated the variability and correlation of the emission in the different energy bands with the fractional variability and the Z-transformed discrete correlation function, as well as its spectral evolution in X-rays and γ rays. Owing to the controversy in the redshift measurements of 1ES 0647+250 reported in the literature, we also estimated its distance in an indirect manner through a comparison of the GeV and TeV spectra from simultaneous observations with Fermi-LAT and MAGIC during the strongest flaring activity detected to date. Moreover, we interpret the SEDs from the four distinct activity states within the framework of one-component and two-component leptonic models, proposing specific scenarios that are able to reproduce the available multi-instrument data. <BR /> Results: We find significant long-term variability, especially in X-rays and VHE γ rays. Furthermore, significant (3‒4σ) correlations were found between the radio, optical, and high-energy (HE) γ-ray fluxes, with the radio emission delayed by about ∼400 days with respect to the optical and γ-ray bands. The spectral analysis reveals a harder-when-brighter trend during the non-flaring state in the X-ray domain. However, no clear patterns were observed for either the enhanced states or the HE (30 MeV &lt; E &lt; 100 GeV) and VHE γ-ray emission of the source. The indirect estimation of the redshift yielded a value of z = 0.45 ± 0.05, which is compatible with some of the values reported in the literature. The SEDs related to the low-activity state and the three flaring states of 1ES 0647+250 can be described reasonably well with the both one-component and two-component leptonic scenarios. However, the long-term correlations indicate the need for an additional radio-producing region located about 3.6 pc downstream from the gamma-ray producing region.

A lower bound on intergalactic magnetic fields from time variability of 1ES 0229+200 from MAGIC and Fermi/LAT observations

Astronomy and Astrophysics · 2023

Context. Extended and delayed emission around distant TeV sources induced by the effects of propagation of γ ray s through the intergalactic medium can be used for the measurement of the intergalactic magnetic field (IGMF). <BR /> Aims: We search for delayed GeV emission from the hard-spectrum TeV γ-ray emitting blazar 1ES 0229+200, with the goal of detecting or constraining the IGMF-dependent secondary flux generated during the propagation of TeV γ rays through the intergalactic medium. <BR /> Methods: We analysed the most recent MAGIC observations over a 5 year time span, and complemented them with historic data of the H.E.S.S. and VERITAS telescopes, along with a 12-year-long exposure of the Fermi/LAT telescope. We used them to trace source evolution in the GeV-TeV band over a decade and a half. We used Monte Carlo simulations to predict the delayed secondary γ-ray flux, modulated by the source variability, as revealed by TeV-band observations. We then compared these predictions for various assumed IGMF strengths to all available measurements of the γ-ray flux evolution. <BR /> Results: We find that the source flux in the energy range above 200 GeV experiences variations around its average on the 14-year time span of observations. No evidence for the flux variability is found in the 1 ‒ 100 GeV energy range accessible to Fermi/LAT. The non-detection of variability due to delayed emission from electromagnetic cascade developing in the intergalactic medium imposes a lower bound of B &gt; 1.8 × 10<SUP>‒17</SUP> G for the long-correlation-length IGMF and B &gt; 10<SUP>‒14</SUP> G for an IGMF of cosmological origin. Though weaker than the one previously derived from the analysis of Fermi/LAT data, this bound is more robust, being based on a conservative intrinsic source spectrum estimate and accounting for the details of source variability in the TeV energy band. We discuss implications of this bound for cosmological magnetic fields that might explain the baryon asymmetry of the Universe.

Search for Gamma-Ray Spectral Lines from Dark Matter Annihilation up to 100 TeV toward the Galactic Center with MAGIC

Physical Review Letters · 2023

Linelike features in TeV γ rays constitute a "smoking gun" for TeV-scale particle dark matter and new physics. Probing the Galactic Center region with ground-based Cherenkov telescopes enables the search for TeV spectral features in immediate association with a dense dark matter reservoir at a sensitivity out of reach for satellite γ -ray detectors, and direct detection and collider experiments. We report on 223 hours of observations of the Galactic Center region with the MAGIC stereoscopic telescope system reaching γ -ray energies up to 100 TeV. We improved the sensitivity to spectral lines at high energies using large-zenith-angle observations and a novel background modeling method within a maximum-likelihood analysis in the energy domain. No linelike spectral feature is found in our analysis. Therefore, we constrain the cross section for dark matter annihilation into two photons to ⟨σ v ⟩≲5 ×10<SUP>-28</SUP> cm<SUP>3</SUP> s<SUP>-1</SUP> at 1 TeV and ⟨σ v ⟩≲1 ×10<SUP>-25</SUP> cm<SUP>3</SUP> s<SUP>-1</SUP> at 100 TeV, achieving the best limits to date for a dark matter mass above 20 TeV and a cuspy dark matter profile at the Galactic Center. Finally, we use the derived limits for both cuspy and cored dark matter profiles to constrain supersymmetric wino models.

cta-observatory/cta-lstchain: v0.9.13 - 2022-01-24

Zenodo · 2023

What's Changed Change conda installation to mamba setup in CI workflow (#1068) @morcuended URGENT: Add change of filter wheels' name in Mongo DB (#1065) @FrancaCassol Contributers @FrancaCassol, @morcuended and @rlopezcoto

Determination of X-ray pulsar geometry with IXPE polarimetry

Nature Astronomy · 2022

Using observations of X-ray pulsar Hercules X-1 by the Imaging X-ray Polarimetry Explorer we report a highly significant (&gt;17σ) detection of the polarization signal from an accreting neutron star. The observed degree of linear polarization of ~10% is far below theoretical expectations for this object, and stays low throughout the spin cycle of the pulsar. Both the degree and angle of polarization exhibit variability with the pulse phase, allowing us to measure the pulsar spin position angle 57(2) deg and the magnetic obliquity 12(4) deg, which is an essential step towards detailed modelling of the intrinsic emission of X-ray pulsars. Combining our results with the optical polarimetric data, we find that the spin axis of the neutron star and the angular momentum of the binary orbit are misaligned by at least ~20 deg, which is a strong argument in support of the models explaining the stability of the observed superorbital variability with the precession of the neutron star.

Gamma-ray observations of MAXI J1820+070 during the 2018 outburst

Monthly Notices of the Royal Astronomical Society · 2022

MAXI J1820+070 is a low-mass X-ray binary with a black hole (BH) as a compact object. This binary underwent an exceptionally bright X-ray outburst from 2018 March to October, showing evidence of a non-thermal particle population through its radio emission during this whole period. The combined results of 59.5 h of observations of the MAXI J1820+070 outburst with the H.E.S.S., MAGIC and VERITAS experiments at energies above 200 GeV are presented, together with Fermi-LAT data between 0.1 and 500 GeV, and multiwavelength observations from radio to X-rays. Gamma-ray emission is not detected from MAXI J1820+070, but the obtained upper limits and the multiwavelength data allow us to put meaningful constraints on the source properties under reasonable assumptions regarding the non-thermal particle population and the jet synchrotron spectrum. In particular, it is possible to show that, if a high-energy (HE) gamma-ray emitting region is present during the hard state of the source, its predicted flux should be at most a factor of 20 below the obtained Fermi-LAT upper limits, and closer to them for magnetic fields significantly below equipartition. During the state transitions, under the plausible assumption that electrons are accelerated up to ~500 GeV, the multiwavelength data and the gamma-ray upper limits lead consistently to the conclusion that a potential HE and very-HE gamma-ray emitting region should be located at a distance from the BH ranging between 10<SUP>11</SUP> and 10<SUP>13</SUP> cm. Similar outbursts from low-mass X-ray binaries might be detectable in the near future with upcoming instruments such as CTA.

Gammapy: Python toolbox for gamma-ray astronomy

Zenodo · 2022

Gammapy analyzes gamma-ray data and creates sky images, spectra and lightcurves, from event lists and instrument response information; it can also determine the position, morphology and spectra of gamma-ray sources. It is used to analyze data from H.E.S.S., Fermi-LAT, HAWC, and the Cherenkov Telescope Array (CTA).

Polarized blazar X-rays imply particle acceleration in shocks

Nature · 2022

Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to around 1 TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnetic field, which can be probed by observations of the polarization of light from the jets. Measurements of the radio to optical polarization—the only range available until now—probe extended regions of the jet containing particles that left the acceleration site days to years earlier<SUP>1-3</SUP>, and hence do not directly explore the acceleration mechanism, as could X-ray measurements. Here we report the detection of X-ray polarization from the blazar Markarian 501 (Mrk 501). We measure an X-ray linear polarization degree Π<SUB>X</SUB> of around 10%, which is a factor of around 2 higher than the value at optical wavelengths, with a polarization angle parallel to the radio jet. This points to a shock front as the source of particle acceleration and also implies that the plasma becomes increasingly turbulent with distance from the shock.

cta-observatory/cta-lstchain: v0.9.8 - 2022-10-24

Zenodo · 2022

What's Changed Recalculate sin_az_tel (#1030) @moralejo Remove numba aot compilation, use njit instead (#1028) @maxnoe Fix disp unit handling, add test (#1029) @maxnoe Contributers @maxnoe, @moralejo and @rlopezcoto

Multiwavelength Observations of the Blazar VER J0521+211 during an Elevated TeV Gamma-Ray State

The Astrophysical Journal · 2022

We report on a long-lasting, elevated gamma-ray flux state from VER J0521+211 observed by VERITAS, MAGIC, and Fermi-LAT in 2013 and 2014. The peak integral flux above 200 GeV measured with the nightly binned light curve is (8.8 ± 0.4) × 10<SUP>-7</SUP> photons m<SUP>-2</SUP> s<SUP>-1</SUP>, or ~37% of the Crab Nebula flux. Multiwavelength observations from X-ray, UV, and optical instruments are also presented. A moderate correlation between the X-ray and TeV gamma-ray fluxes was observed, and the X-ray spectrum appeared harder when the flux was higher. Using the gamma-ray spectrum and four models of the extragalactic background light (EBL), a conservative 95% confidence upper limit on the redshift of the source was found to be z ≤ 0.31. Unlike the gamma-ray and X-ray bands, the optical flux did not increase significantly during the studied period compared to the archival low-state flux. The spectral variability from optical to X-ray bands suggests that the synchrotron peak of the spectral energy distribution (SED) may become broader during flaring states, which can be adequately described with a one-zone synchrotron self-Compton model varying the high-energy end of the underlying particle spectrum. The synchrotron peak frequency of the SED and the radio morphology of the jet from the MOJAVE program are consistent with the source being an intermediate-frequency-peaked BL Lac object.

Gammapy: Python toolbox for gamma-ray astronomy

Zenodo · 2022

Gammapy analyzes gamma-ray data and creates sky images, spectra and lightcurves, from event lists and instrument response information; it can also determine the position, morphology and spectra of gamma-ray sources. It is used to analyze data from H.E.S.S., Fermi-LAT, and the Cherenkov Telescope Array (CTA).

Kinematics of Parsec-scale Jets of Gamma-Ray Blazars at 43 GHz during 10 yr of the VLBA-BU-BLAZAR Program

The Astrophysical Journal Supplement Series · 2022

We analyze the parsec-scale jet kinematics from 2007 June to 2018 December of a sample of γ-ray bright blazars monitored roughly monthly with the Very Long Baseline Array (VLBA) at 43 GHz under the VLBA-BU-BLAZAR program. We implement a novel piecewise linear fitting method to derive the kinematics of 521 distinct emission knots from a total of 3705 total intensity images in 22 quasars, 13 BL Lacertae objects, and 3 radio galaxies. Apparent speeds of these components range from 0.01c to 78c, and 18.6% of knots (other than the "core") are quasi-stationary. One-fifth of moving knots exhibit nonballistic motion, with acceleration along the jet within 5 pc of the core (projected) and deceleration farther out. These accelerations occur mainly at locations coincident with quasi-stationary features. We calculate the physical parameters of 273 knots with statistically significant motion, including their Doppler factors, Lorentz factors, and viewing angles. We determine the typical values of these parameters for each jet and the average for each subclass of active galactic nuclei. We investigate the variability of the position angle of each jet over the 10 yr of monitoring. The fluctuations in position of the quasi-stationary components in radio galaxies tend to be parallel to the jet, while no directional preference is seen in the components of quasars and BL Lacertae objects. We find a connection between γ-ray states of blazars and their parsec-scale jet properties, with blazars with brighter 43 GHz cores typically reaching higher γ-ray maxima during flares.

cta-observatory/ctapipe: v0.14.0 – 2022-04-14

Zenodo · 2022

What's Changed since v0.13.0 This release fixes an important issue in the stereo reconstruction: the quality criteria for individual telescope events to enter into the stereo reconstruction were not applied correctly. Fixing this required an API change. The quality criteria are now part of the Reconstructor as StereoQualityQuery, not part of the ShowerProcessor any more. As part of this change, we also added back support for using the HillasIntersection additionally to the HillasReconstructor. The second large change is the addition of support for two better formats for config files. Additionally to the json supported before, ctapipe now also supports yaml and toml configuration files and tools support giving multiple configuration files. Fix bug in ShowerProcessor / HillasReconstructor not using QualityQuery correctly, add support for HillasIntersection in ShowerProcessor (#1875) @maxnoe allow YAML and TOML config files and multiple configs (#1856) @kosack Remove parallel=True for neighbor_waveform_sum, fixes #1532 (#1878) @maxnoe Add switch to ImageProcessor to enable/disable ImageModifier (#1873) @maxnoe Contributors @kosack and @maxnoe

cta-observatory/cta-lstchain: v0.9.6 - 2022-04-13

Zenodo · 2022

What's Changed Update readme prior to new release (#968) @rlopezcoto Require at least ctapipe_io_lst 0.18.1 (#967) @maxnoe Contributers @maxnoe and @rlopezcoto

cta-observatory/cta-lstchain: v0.9.5 – 2022-04-12

Zenodo · 2022

What's Changed Reduce size of random forests in standard config (#961) @vuillaut Use setup-miniconda in CI (#966) @maxnoe Remove hard-coded arguments in the energy-dependent cuts functions (#965) @chaimain Allow selecting N samples for training and testing (#949) @vuillaut Update ctapipe_io_lst and add --flatfield-heuristic switch to relevant programs (#950) @maxnoe Fix missing f-strings in io (#952) @maxnoe Remove hard coded parameters in analysis_muon_event and make it usable with any telescope id. (#944) @gabemery set joblib compress to 3 (#947) @vuillaut Contributers @chaimain, @gabemery, @maxnoe, @moralejo and @vuillaut

Author Correction: Proton acceleration in thermonuclear nova explosions revealed by gamma rays

Nature Astronomy · 2022

Proton acceleration in thermonuclear nova explosions revealed by gamma rays

Nature Astronomy · 2022

Classical novae are cataclysmic binary star systems in which the matter of a companion star is accreted on a white dwarf<SUP>1,2</SUP>. Accumulation of hydrogen in a layer eventually causes a thermonuclear explosion on the surface of the white dwarf<SUP>3</SUP>, brightening the white dwarf to ~10<SUP>5</SUP> solar luminosities and triggering ejection of the accumulated matter. Novae provide the extreme conditions required to accelerate particles, electrons or protons, to high energies. Here we present the detection of gamma rays by the MAGIC telescopes from the 2021 outburst of RS Ophiuchi, a recurrent nova with a red giant companion, which allowed us to accurately characterize the emission from a nova in the 60 GeV to 250 GeV energy range. The theoretical interpretation of the combined Fermi LAT and MAGIC data suggests that protons are accelerated to hundreds of gigaelectronvolts in the nova shock. Such protons should create bubbles of enhanced cosmic ray density, of the order of 10 pc, from the recurrent novae.

cta-observatory/cta-lstchain: v0.9.4

Zenodo · 2022

What's Changed Improve DL1 check treatment of problematic data (#939) @moralejo Update minimum required ctapipe_io_lst to 0.16.1 (#938) @maxnoe Update of a source-dependent config file (#937) @SeiyaNozaki Fixes in DL1 data check (#932) @moralejo Improvements of data selection notebook (#935) @moralejo Added notebook for the selection of runs for analysis (#934) @moralejo Contributers @SeiyaNozaki, @maxnoe, @moralejo, @rlopezcoto and Franca Cassol

Investigating the Blazar TXS 0506+056 through Sharp Multiwavelength Eyes During 2017-2019

The Astrophysical Journal · 2022

The blazar TXS 0506+056 got into the spotlight of the astrophysical community in 2017 September, when a high-energy neutrino detected by IceCube (IceCube-170922A) was associated at the 3σ level with a γ-ray flare from this source. This multi-messenger photon-neutrino association remains, as per today, the most significant association ever observed. TXS 0506+056 was a poorly studied object before the IceCube-170922A event. To better characterize its broadband emission, we organized a multiwavelength campaign lasting 16 months (2017 November to 2019 February), covering the radio band (Metsähovi, OVRO), the optical/UV (ASAS-SN, KVA, REM, Swift/UVOT), the X-rays (Swift/XRT, NuSTAR), the high-energy γ rays (Fermi/LAT), and the very high-energy (VHE) γ rays (MAGIC). In γ rays, the behavior of the source was significantly different from the behavior in 2017: MAGIC observations show the presence of flaring activity during 2018 December, while the source only shows an excess at the 4σ level during the rest of the campaign (74 hr of accumulated exposure); Fermi/LAT observations show several short (on a timescale of days to a week) flares, different from the long-term brightening of 2017. No significant flares are detected at lower energies. The radio light curve shows an increasing flux trend that is not seen in other wavelengths. We model the multiwavelength spectral energy distributions in a lepto-hadronic scenario, in which the hadronic emission emerges as Bethe-Heitler and pion-decay cascade in the X-rays and VHE γ rays. According to the model presented here, the 2018 December γ-ray flare was connected to a neutrino emission that was too brief and not bright enough to be detected by current neutrino instruments.

gammapy/gammapy: v.0.19

Zenodo · 2021

A Python package for gamma-ray astronomy

cta-observatory/ctapipe: v0.12.0

Zenodo · 2021

What's Changed since v0.11.0 The main changes in this release are related to expanding the data model to include DL2 and the computation of DL2 parameters. This also required changing the units of some DL1 parameters from the CameraFrame to the TelescopeFrame to make them easier to compare between telescopes, which includes an overhaul of how the HillasReconstructor works (supporting both frames as input), and adds include a new Component ShowerProcessor that generates DL2(a) information. The data model was expanded to include DL2 and to support a more general InstrumentDescription serialization. DL1Writer was refactored into a general DataWriter that writes the full data model. TableLoader provides a user interface to load all events at once from ctapipe output files, joining them as needed, which is useful for benchmarking. The ctapipe-stage1 tool was removed and replaced with the more general ctapipe-process, which can now handle any transformation from R0 to DL2a, depending on what configuration file it is given. A ctapipe-quickstarttool was added to generate some starting config files, and a ctapipe-fileinfo tool to show file header information for any files written by ctapipe. New Features Add flag to use camera frame for ctapipe-process (#1790) @maxnoe TableLoader (#1771) @maxnoe Add flags to enforce recomputation of dl1/dl2 (#1776) @LukasNickel ctapipe-quickstart tool (#1774) @kosack Add condition option to read_table (#1769) @maxnoe Implement allow_none=False for Path trait, fixes #1759 (#1760) @maxnoe ctapipe-fileinfo (#1756) @kosack Configurable contact information for DataWriter (#1757) @maxnoe Generalize image converter for square pixels (#1728) @LukasNickel Implement ctapipe-process (#1726) @kosack Add support for calibration scale and shift in R1 (#1749) @HealthyPear Direction reconstruction (also) from Telescope Frame (#1408) @HealthyPear Add no-flags, add short aliases, make aliases more consistent (#1748) @nbiederbeck Adapt subarray for 'allowed_tels' option in merger_tool (#1736) @Hckjs Implement a shower processor (#1675) @HealthyPear DL2 data model and output (#1673) @kosack Allow regexp in table name for TableWriter.exclude() (#1717) @kosack Shortform aliases (#1713) @maxnoe implements add_column_transform_regexp feature (#1690) @kosack Use iminuit>=2 (#1714) @nbiederbeck Breaking Changes Both of these change the data model, but the code remains backward-compatible. Use indices for camera / optics lookup in from_hdf / to_hdf (#1426) @maxnoe Hillas parameters in telescope frame (optionally) (#1591) @LukasNickel ctapipe-stage1 refactored into ctapipe-process (#1726) Performance Improvements Do not guess pixel area when transforming (#1794) @maxnoe Improve performance of calc_pixel_neighbors, fixes #1791 (#1792) @maxnoe Precompile table regex for column exclusion, fixes #1719 (#1720) @maxnoe Bug Fixes and Refactoring Compare to numpy integer (#1801) @LukasNickel Fix negative vmin/vmax raising error with mpl3.5, fixes #1805 (#1807) @maxnoe Fix display dl1 (#1795) @maxnoe Fix writing FITS in fileinfo for unicode (#1789) @maxnoe Fix TelescopeParameter for traitlets 5.1 (#1784) @maxnoe Remove unnecessary is_simulation for ImageProcessor (#1773) @maxnoe Remove debug prints, fixes #1764 (#1765) @maxnoe Remove redundant test code in simtel event source test code (#1747) @maxnoe Change REF_WIDTH to REFWIDTH to fit into 8 characters for FITS, fixes #1452 (#1753) @maxnoe Rename function. Fix #1733. (#1734) @nbiederbeck Fix ctapipe-merge with --allowed-tels (#1712) @nbiederbeck Set errordef for taubin fit (#1716) @maxnoe Documentation, Testing, and Build updates a big change is that tests are now parallelizable, leading to faster results from the CI system. Removing unwanted quotes for code block in readme (#1808) @vuillaut Add note about PYTHONNOUSERSITE to README (#1806) @maxnoe Fix deprecated keys in setup.cfg (#1804) @maxnoe Fix several warnings during tests (#1787) @maxnoe Unit tests: fix mutated camera geometry fixtures (#1793) @maxnoe Build docs using two cores, fix name of step (#1781) @maxnoe Do not use sphinx action for building docs (#1780) @maxnoe include stage1_config.json in package (#1752) @nbiederbeck Switch on doctests in the CI (#1731) @maxnoe Add tests for download functions (#1763) @maxnoe Fix docs for Path (#1758) @maxnoe Use pypa/build for packaging (#1725) @maxnoe Cache the test data (#1746) @maxnoe Improve dl1 fixtures and fix test concurrency issues (#1741) @maxnoe Parallel tests (#1740) @maxnoe Pyflakes in CI (#1735) @maxnoe Add citing information for version 0.11 (#1710) @LukasNickel Contributors @Hckjs, @HealthyPear, @LukasNickel, @kosack, @maxnoe, @nbiederbeck and @vuillaut

Investigation of the correlation patterns and the Compton dominance variability of Mrk 421 in 2017

Astronomy and Astrophysics · 2021

<BR /> Aims: We present a detailed characterisation and theoretical interpretation of the broadband emission of the paradigmatic TeV blazar Mrk 421, with a special focus on the multi-band flux correlations. <BR /> Methods: The dataset has been collected through an extensive multi-wavelength campaign organised between 2016 December and 2017 June. The instruments involved are MAGIC, FACT, Fermi-LAT, Swift, GASP-WEBT, OVRO, Medicina, and Metsähovi. Additionally, four deep exposures (several hours long) with simultaneous MAGIC and NuSTAR observations allowed a precise measurement of the falling segments of the two spectral components. <BR /> Results: The very-high-energy (VHE; E &gt; 100 GeV) gamma rays and X-rays are positively correlated at zero time lag, but the strength and characteristics of the correlation change substantially across the various energy bands probed. The VHE versus X-ray fluxes follow different patterns, partly due to substantial changes in the Compton dominance for a few days without a simultaneous increase in the X-ray flux (i.e., orphan gamma-ray activity). Studying the broadband spectral energy distribution (SED) during the days including NuSTAR observations, we show that these changes can be explained within a one-zone leptonic model with a blob that increases its size over time. The peak frequency of the synchrotron bump varies by two orders of magnitude throughout the campaign. Our multi-band correlation study also hints at an anti-correlation between UV-optical and X-ray at a significance higher than 3σ. A VHE flare observed on MJD 57788 (2017 February 4) shows gamma-ray variability on multi-hour timescales, with a factor ten increase in the TeV flux but only a moderate increase in the keV flux. The related broadband SED is better described by a two-zone leptonic scenario rather than by a one-zone scenario. We find that the flare can be produced by the appearance of a compact second blob populated by high energetic electrons spanning a narrow range of Lorentz factors, from γ'<SUB>min</SUB>=2×10<SUP>4</SUP> to γ'<SUB>max</SUB>=6×10<SUP>5</SUP>. <P />Light curves and spectral energy distributions data are only available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr/">cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/655/A89">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/655/A89</A>

cta-observatory/ctapipe: v0.11.0

Zenodo · 2021

What's Changed since v0.10.5 This is a major upgrade due to the removal of some deprecated and internally unused modules and functions. We also changed the default of CameraCalibrator.apply_waveform_timeshift and invalid configuration options will now raise an error, so some configs might need updates. One major change is that the event sources now will adapt the subarray and trigger info if allowed_tels is used. The default of Breaking changes Change default of CameraCalibrator.apply_waveform_timeshift to False (#1692) @maxnoe Consistent use of allowed_tels (#1697) @LukasNickel Raise error in CameraReadout.from_name if no corresponding file can be found (#1703) @maxnoe Remove unused io modules, fixes #1527 (#1693) @maxnoe Change definition of concentration_cog, fixes #1571 (#1572) @maxnoe Transform warning into error for wrong config (#1658) @maxnoe Remove charge resolution tools, fixes #1634 (#1646) @maxnoe New features Store package versions in provenance (#1685) @maxnoe Store used simtel R1 calibration in event.monitoring (#1691) @maxnoe Add start, stop, step to read_table (#1684) @maxnoe Support http(s):// and dataset:// for path traitlet (#1638) @maxnoe Add brightest_island function (#1587) @maxnoe Allow specifying extractors per telescope, fixes #1660 (#1661) @maxnoe Fix bugs in TwoPassWindowSum and define better unit-test (#1698) @HealthyPear Use new random API (#1708) @maxnoe Pin sphinx to 3.5 for now (#1709) @maxnoe Allow appending of rows to existing tables in HDF5TableWriter, fixes #1655 (#1657) @maxnoe Bug fixes / maintenance Update documentation Makefile (#1705) @HealthyPear Fix hillas intersection for "missing" telescope ids, fixes #1635 (#1702) @maxnoe Update mailmap (#1700) @maxnoe Fix two numpy deprecation warnings (#1704) @maxnoe Fix pixel picking in mpl display, add test (#1701) @maxnoe Fix dl1source (#1696) @LukasNickel Make sure hdf5 file is closed when necessary in read_table, fixes #1694 (#1695) @maxnoe Treat sphinx warnings as erros, fix deploy workflow name (#1686) @maxnoe Do not use numba cache for lts_linear_fit (#1680) @maxnoe Fix syntax typo in call of TwoPassWindowSum with 2ndPass disabled (#1681) @HealthyPear Remove DL1Writer.setup, just properly initialize in init (#1667) @maxnoe Fix codecov missing coverage for cli tools (#1652) @maxnoe Use tqdm.auto (#1659) @maxnoe Require at least astropy 4.0.5, drop restriction to h5py 2.x (#1676) @maxnoe Fix some documentation warnings (#1677) @HealthyPear Create cache dir only when needed (#1670) @maxnoe Use conda-forge channel in README and add some notes (#1671) @maxnoe Update dl1writer.py docstring (#1666) @vuillaut Fix time_delta cleaning (#1656) @LukasNickel Update badge to 0.10.5 (#1650) @maxnoe Generalize URL for downloading test data (#1651) @HealthyPear Update README.rst (#1647) @orelgueta Contributors @HealthyPear, @LukasNickel, @maxnoe, @orelgueta and @vuillaut

Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign

The Astrophysical Journal · 2021

In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass ∼6.5 × 10<SUP>9</SUP>M<SUB>☉</SUB>. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87's spectrum. We can exclude that the simultaneous γ-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the γ-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.

cta-observatory/ctapipe: v0.10.5

Zenodo · 2021

What's Changed since v0.10.4 fix a bug in twopass extractor where the timing slope was wrong (#1639) @kosack Add missing f-string in dl1-merge to see broken filenames, fixes #1643 (#1644) @Hckjs Make sure each test gets a copy of event/subarray (#1637) @maxnoe Contributors @Hckjs, @kosack and @maxnoe

Sensitivity of the Cherenkov Telescope Array for probing cosmology and fundamental physics with gamma-ray propagation

Journal of Cosmology and Astroparticle Physics · 2021

The Cherenkov Telescope Array (CTA), the new-generation ground-based observatory for γ astronomy, provides unique capabilities to address significant open questions in astrophysics, cosmology, and fundamental physics. We study some of the salient areas of γ cosmology that can be explored as part of the Key Science Projects of CTA, through simulated observations of active galactic nuclei (AGN) and of their relativistic jets. Observations of AGN with CTA will enable a measurement of γ absorption on the extragalactic background light with a statistical uncertainty below 15% up to a redshift z=2 and to constrain or detect γ halos up to intergalactic-magnetic-field strengths of at least 0.3 pG . Extragalactic observations with CTA also show promising potential to probe physics beyond the Standard Model. The best limits on Lorentz invariance violation from γ astronomy will be improved by a factor of at least two to three. CTA will also probe the parameter space in which axion-like particles could constitute a significant fraction, if not all, of dark matter. We conclude on the synergies between CTA and other upcoming facilities that will foster the growth of γ cosmology.

Sensitivity of the Cherenkov Telescope Array to a dark matter signal from the Galactic centre

Journal of Cosmology and Astroparticle Physics · 2021

We provide an updated assessment of the power of the Cherenkov Telescope Array (CTA) to search for thermally produced dark matter at the TeV scale, via the associated gamma-ray signal from pair-annihilating dark matter particles in the region around the Galactic centre. We find that CTA will open a new window of discovery potential, significantly extending the range of robustly testable models given a standard cuspy profile of the dark matter density distribution. Importantly, even for a cored profile, the projected sensitivity of CTA will be sufficient to probe various well-motivated models of thermally produced dark matter at the TeV scale. This is due to CTA's unprecedented sensitivity, angular and energy resolutions, and the planned observational strategy. The survey of the inner Galaxy will cover a much larger region than corresponding previous observational campaigns with imaging atmospheric Cherenkov telescopes. CTA will map with unprecedented precision the large-scale diffuse emission in high-energy gamma rays, constituting a background for dark matter searches for which we adopt state-of-the-art models based on current data. Throughout our analysis, we use up-to-date event reconstruction Monte Carlo tools developed by the CTA consortium, and pay special attention to quantifying the level of instrumental systematic uncertainties, as well as background template systematic errors, required to probe thermally produced dark matter at these energies.

cta-observatory/ctapipe: v0.9.1

Zenodo · 2020

This is a primarily a bugfix release, other than one new feature: ctapipe.io.astropy_helpers.h5_table_to_astropy() What's Changed since v0.9.0 Bug Fixes Fix loss of performance in TwoPassWindowSum (#1442) @HealthyPear Fix rewrite subarray (#1446) @maxnoe Return nan for non-invertible matrices (#1439) @maxnoe Add fits option to meta.to_dict, silences warnings (#1433) @maxnoe Features add function to get astropy table from ctapipe hdf5 file (#1427) @kosack other replace Release Drafter App with Workflow (#1454) @kosack Contributors @HealthyPear, @kosack and @maxnoe

Unraveling the Complex Behavior of Mrk 421 with Simultaneous X-Ray and VHE Observations during an Extreme Flaring Activity in 2013 April

The Astrophysical Journal Supplement Series · 2020

We report on a multiband variability and correlation study of the TeV blazar Mrk 421 during an exceptional flaring activity observed from 2013 April 11 to 19. The study uses, among others, data from GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT), Swift, Nuclear Spectroscopic Telescope Array (NuSTAR), Fermi Large Area Telescope, Very Energetic Radiation Imaging Telescope Array System (VERITAS), and Major Atmospheric Gamma Imaging Cherenkov (MAGIC). The large blazar activity and the 43 hr of simultaneous NuSTAR and MAGIC/VERITAS observations permitted variability studies on 15 minute time bins over three X-ray bands (3-7 keV, 7-30 keV, and 30-80 keV) and three very-high-energy (VHE; &gt;0.1 TeV) gamma-ray bands (0.2-0.4 TeV, 0.4-0.8 TeV, and &gt;0.8 TeV). We detected substantial flux variations on multi-hour and sub-hour timescales in all of the X-ray and VHE gamma-ray bands. The characteristics of the sub-hour flux variations are essentially energy independent, while the multi-hour flux variations can have a strong dependence on the energy of the X-rays and the VHE gamma-rays. The three VHE bands and the three X-ray bands are positively correlated with no time lag, but the strength and characteristics of the correlation change substantially over time and across energy bands. Our findings favor multi-zone scenarios for explaining the achromatic/chromatic variability of the fast/slow components of the light curves, as well as the changes in the flux-flux correlation on day-long timescales. We interpret these results within a magnetic reconnection scenario, where the multi-hour flux variations are dominated by the combined emission from various plasmoids of different sizes and velocities, while the sub-hour flux variations are dominated by the emission from a single small plasmoid moving across the magnetic reconnection layer. <SUP>*</SUP> Contact MAGIC Collaboration (contact.magic@mpp.mpg.de) for queries. Corresponding authors are D. Paneque, A. Babic, J. Finke, T. Hassan, and M. Petropoulou.

cta-observatory/ctapipe: v0.8.0

Zenodo · 2020

Main features/changes: ctapipe-stage1-process: standard Tool to generate DL1 outputs, along with many related changes to the internal data model Improved/refactored calibration and ImageExtraction code (including a 2-Pass image extractor using time gradient information) Improved/refactored Muon reconstruction code SubarrayDescription is now accessed directly from the EventSource, rather than the event itself, and now includes a CameraDescription class that aggregates CameraGeometry and CameraReadout. Toy model EventSource and Waveform generators improved configuration with per-telescope (Type or ID) parameters plus many more! What's Changed since v0.7.0 Improve performance of concentration by 2x by stripping units internally (#1350) @MaxNoe Use precomputed mean/std to compute skewness and kurtosis (#1349) @MaxNoe Update mailmap (#1346) @MaxNoe last fixes for release of v0.8 (#1348) @kosack reformat with black (#1345) @kosack add vitables to user environment (#1344) @kosack Make sure unit is really a unit, use fits format (#1343) @MaxNoe remove mcheader attributes accidentally added in merge (#1342) @kosack DL1 writer tool (ctapipe-stage1-process) (#1163) @kosack fix bug when ignore_telescopes was used in sim_telarray (#1338) @yrenier fix bug where image was 64-bit in case where no extraction is needed (#1339) @kosack Fix examples, fixes #1331 (#1332) @MaxNoe Make sure extractors return 32bit image / peak_time (#1336) @MaxNoe use float32 for images, and validate (#1329) @kosack Fix bokeh to version 1 (#1326) @MaxNoe Fix hillas parameters divide by 0 warning, fixes #1327 (#1328) @MaxNoe Rename muon containers to make naming more consistent (#1324) @MaxNoe Add telescope trigger times, move event type to trigger (#1322) @MaxNoe Make input_url possible as positional argument, cleanup factory tests (#1321) @MaxNoe Calculate ring containment analytically (#1256) @MaxNoe Add more properties to EventSource, implement for SimTelEventSource, fixes #1286 (#1313) @MaxNoe Update tests with new ctapipe-extra, remove hessio remnants (#1318) @MaxNoe Fix parent propagation in hdf writer (#1320) @MaxNoe 2-Pass waveform integration à-la-CTA/MARS (#1215) @HealthyPear Calculate muon image parameters (#1316) @MaxNoe Replace more usages of Unicode with Path (#1319) @MaxNoe Fix writing of default image parameters container (#1311) @MaxNoe Add missing config, parent to hdfwriter (#1312) @MaxNoe Remove empty or misleading files (#1314) @MaxNoe Hdf5 column order (#1308) @MaxNoe Container Improvements and Cleanup for DL1 (#1301) @kosack Make arguments to enum_trait consistent, rename to create_class_enum_trait, fixes #1282 (#1306) @MaxNoe Add array pointing to pointing, use .tel map (#1303) @MaxNoe remove HESSIOEventSource (#1304) @kosack Rename coordinates of NominalFrame and TelescopeFrame, fixes #1260 (#1299) @MaxNoe Remove deprecated instrument from event container (#1294) @MaxNoe Toy source (#1298) @MaxNoe Use pointing container in examples and docs (#1296) @MaxNoe Remove old, untested plotting code (#1297) @MaxNoe Update documentation for CameraCalibrator and ImageExtractor (#1283) @watsonjj Store filters and use for new datasets in HDFWriter (#1285) @MaxNoe Added tailcuts data volume reducer (#1121) @Hckjs Drop dead code in simtel event source (#1288) @MaxNoe Muon intensity improvements (#1261) @MaxNoe Remove sst1m specific containers (#1292) @MaxNoe Update travis badge to new travis link (#1291) @MaxNoe Fix calculation of pixel area guessing (#1290) @MaxNoe Path improvements (#1284) @MaxNoe CTA Reference Metadata handling (#1221) @kosack add option to use effective focal length, fixes #1223 (#1262) @kosack Make ImageExtractor TelescopeComponent, remove duplicated code (#1280) @MaxNoe Add BoolTelescopeParameter, expose all options of TailCutsCleaner (#1281) @MaxNoe Remove unused and untested muon fitting code (#1254) @MaxNoe Fix bug in ImageExtractor and WaveformModel concerning waveform sampling rate (#1276) @watsonjj add support of float16 in PYTABLES_TYPE_MAP (#1273) @vuillaut Ignore unmentioned references (#1269) @MaxNoe Muon refactoring (#1253) @MaxNoe Fix tests for Tools, fixes #1214 (#1027) @dneise Move containers from io to base (#1267) @MaxNoe Fix input url handling in display_dl1 (#1251) @MaxNoe Test with 3.8 (#1174) @MaxNoe Replace muon container defaults with nan, remove redundant fields (#1248) @MaxNoe Waveform toy model (#1244) @watsonjj Various bug fixes and improvements in muon ring analysis (#1245) @moralejo fix codacy badge (use cta-observatory org) (#1250) @kosack Add pyproject toml (#1246) @MaxNoe Created CameraDescription refactor CameraGeometry (#1241) @watsonjj Add a Selector class for keeping track of quality cuts (#1207) @kosack ensure command-line args have higher precedence than config options (#1168) @kosack Cleaning for biggest cluster (#1131) @HealthyPear Improvements to ImageExtractor wrt integration_correction and sampling rate (#1242) @watsonjj Fix hillas width 0, fixes #772 (#1240) @MaxNoe Replace np.power with power operator, fixes #1028 (#1239) @MaxNoe Move pulse shape attributes from mc container to CameraGeometry (#1227) @watsonjj Do not mutate geometry in camera display, fixes #1190 (#1237) @MaxNoe Convert config file argument to path type, fixes #1196 (#1236) @MaxNoe Improvement of integration_correction testing, implementation, and docstring (#1233) @watsonjj Make subarray a required argument to CameraCalibrator and ImageExtractor (#1228) @watsonjj Remove extract_pulse_time_around_peak (#1234) @watsonjj Fix bokeh viewer for new bokeh version (#1232) @MaxNoe Fix array display for mpl 3.2, fixes #1229 (#1231) @MaxNoe Add Monitoring container to MC data (#1115) @FrancaCassol Fix TelescopeParameter config, bug #1216 (#1218) @kosack fix #1217 by removing unused _hyperbinning() function (#1219) @kosack bump astropy version requirement to allow v4.0 (#1213) @kosack Fixes for astropy 4.x (#1212) @kosack Pypi upload (#1208) @MaxNoe add Taubin fit (#1154) @momorning Add guessing MAGIC telescope (#1206) @adonini configurable image cleaners (#1201) @kosack remove deprecation warning: replace log.warn --> log.warning (#1204) @dneise add helper function, might be nice for parametrized tests (#1203) @dneise Create CODEOWNERS (#1151) @watsonjj colorbar default axes as CameraDisplay.axes (#1193) @vuillaut Add directions to GroundFrame docstring as described in #1183 (#1185) @nbiederbeck fix typo in contributors for release drafter (#1189) @vuillaut Update ImageExtractors to utilise TelescopeParameters (#1155) @watsonjj Extra TelescopeParameter use cases (#1175) @watsonjj Return global value from TelescopeParameterLookup when passed None (#1173) @watsonjj Scalar default values in TelescopeParameter (#1171) @watsonjj Fixes for CameraCalibrator and extract_around_peak (#1172) @watsonjj Remove inheritance on list for TelescopeParameterList (#1162) @watsonjj Add application of DL1 charge calibration to CameraCalibrator (#1160) @watsonjj Enable access to subarray information from file before event loop (#1157) @watsonjj Add sampling_rate to instruments description (#1142) @watsonjj Combine sample loops (#1146) @watsonjj Update eventio version in setup.py (#1169) @watsonjj Move gain selection from CameraCalibrator to EventSource (#1167) @watsonjj Add resolving to the TelescopeParameter (#1158) @watsonjj Pointing container (#1141) @vuillaut fixed bug in TelescopeParameterResolver (#1156) @kosack Add instructions for Slack (#1152) @watsonjj increase nbsphinx cell timeout (#1153) @kosack added DOI for release v0.7.0 (#1150) @kosack Improvements to Container classes (#1123) @kosack Improve extract_pulse_time_around_peak to ignore negative samples (#1143) @watsonjj Remove 3-dimension extractors (#1144) @watsonjj Add provenance.log to .gitignore (#1145) @watsonjj Also test ctapipe with a pure pip installation (no conda) (#1137) @MaxNoe Fix/core tool (#1124) @kosack Fix unit error in SkewedGaussian toymodel (#1120) @MaxNoe TelescopeParameter Trait (#1129) @kosack Use astropys own version of quantity_approx (#1128) @MaxNoe Ci warnings (#1136) @MaxNoe Fix dtype of island labels (#1134) @MaxNoe Use count_nonzero instead of sum for bool arrays (#1133) @MaxNoe a few small random cleanups (#1126) @kosack ensure gain selection returns uint8, not 64-bit int (#1125) @kosack timing_parameters cleaning mask (#1119) @vuillaut Contributors @FrancaCassol, @Hckjs, @HealthyPear, @MaxNoe, @adonini, @dneise, @kosack, @momorning, @moralejo, @nbiederbeck, @vuillaut, @watsonjj and @yrenier

Observation of inverse Compton emission from a long γ-ray burst

Nature · 2019

Long-duration γ-ray bursts (GRBs) originate from ultra-relativistic jets launched from the collapsing cores of dying massive stars. They are characterized by an initial phase of bright and highly variable radiation in the kiloelectronvolt-to-megaelectronvolt band, which is probably produced within the jet and lasts from milliseconds to minutes, known as the prompt emission<SUP>1,2</SUP>. Subsequently, the interaction of the jet with the surrounding medium generates shock waves that are responsible for the afterglow emission, which lasts from days to months and occurs over a broad energy range from the radio to the gigaelectronvolt bands<SUP>1-6</SUP>. The afterglow emission is generally well explained as synchrotron radiation emitted by electrons accelerated by the external shock<SUP>7-9</SUP>. Recently, intense long-lasting emission between 0.2 and 1 teraelectronvolts was observed from GRB 190114C<SUP>10,11</SUP>. Here we report multi-frequency observations of GRB 190114C, and study the evolution in time of the GRB emission across 17 orders of magnitude in energy, from 5 × 10<SUP>-6</SUP> to 10<SUP>12</SUP> electronvolts. We find that the broadband spectral energy distribution is double-peaked, with the teraelectronvolt emission constituting a distinct spectral component with power comparable to the synchrotron component. This component is associated with the afterglow and is satisfactorily explained by inverse Compton up-scattering of synchrotron photons by high-energy electrons. We find that the conditions required to account for the observed teraelectronvolt component are typical for GRBs, supporting the possibility that inverse Compton emission is commonly produced in GRBs.

All-sky Medium Energy Gamma-ray Observatory: Exploring the Extreme Multimessenger Universe

Bulletin of the American Astronomical Society · 2019

The All-sky Medium Energy Gamma-ray Observatory (AMEGO) is a probe class mission concept that will provide essential contributions to multimessenger astrophysics in the late 2020s and beyond. AMEGO combines high sensitivity in the 200 keV to 10 GeV energy range with a wide field of view, good spectral resolution, and polarization sensitivity.

Monte Carlo studies for the optimisation of the Cherenkov Telescope Array layout

Astroparticle Physics · 2019

The Cherenkov Telescope Array (CTA) is the major next-generation observatory for ground-based very-high-energy gamma-ray astronomy. It will improve the sensitivity of current ground-based instruments by a factor of five to twenty, depending on the energy, greatly improving both their angular and energy resolutions over four decades in energy (from 20 GeV to 300 TeV). This achievement will be possible by using tens of imaging Cherenkov telescopes of three successive sizes. They will be arranged into two arrays, one per hemisphere, located on the La Palma island (Spain) and in Paranal (Chile). We present here the optimised and final telescope arrays for both CTA sites, as well as their foreseen performance, resulting from the analysis of three different large-scale Monte Carlo productions.

Compact radio emission indicates a structured jet was produced by a binary neutron star merger

Science · 2019

The binary neutron star merger event GW170817 was detected through both electromagnetic radiation and gravitational waves. Its afterglow emission may have been produced by either a narrow relativistic jet or an isotropic outflow. High-spatial-resolution measurements of the source size and displacement can discriminate between these scenarios. We present very-long-baseline interferometry observations, performed 207.4 days after the merger by using a global network of 32 radio telescopes. The apparent source size is constrained to be smaller than 2.5 milli-arc seconds at the 90% confidence level. This excludes the isotropic outflow scenario, which would have produced a larger apparent size, indicating that GW170817 produced a structured relativistic jet. Our rate calculations show that at least 10% of neutron star mergers produce such a jet.

Blazar spectral variability as explained by a twisted inhomogeneous jet

Nature · 2017

Blazars are active galactic nuclei, which are powerful sources of radiation whose central engine is located in the core of the host galaxy. Blazar emission is dominated by non-thermal radiation from a jet that moves relativistically towards us, and therefore undergoes Doppler beaming. This beaming causes flux enhancement and contraction of the variability timescales, so that most blazars appear as luminous sources characterized by noticeable and fast changes in brightness at all frequencies. The mechanism that produces this unpredictable variability is under debate, but proposed mechanisms include injection, acceleration and cooling of particles, with possible intervention of shock waves or turbulence. Changes in the viewing angle of the observed emitting knots or jet regions have also been suggested as an explanation of flaring events and can also explain specific properties of blazar emission, such as intra-day variability, quasi-periodicity and the delay of radio flux variations relative to optical changes. Such a geometric interpretation, however, is not universally accepted because alternative explanations based on changes in physical conditions—such as the size and speed of the emitting zone, the magnetic field, the number of emitting particles and their energy distribution—can explain snapshots of the spectral behaviour of blazars in many cases. Here we report the results of optical-to-radio-wavelength monitoring of the blazar CTA 102 and show that the observed long-term trends of the flux and spectral variability are best explained by an inhomogeneous, curved jet that undergoes changes in orientation over time. We propose that magnetohydrodynamic instabilities or rotation of the twisted jet cause different jet regions to change their orientation and hence their relative Doppler factors. In particular, the extreme optical outburst of 2016-2017 (brightness increase of six magnitudes) occurred when the corresponding emitting region had a small viewing angle. The agreement between observations and theoretical predictions can be seen as further validation of the relativistic beaming theory.

Multi-messenger Observations of a Binary Neutron Star Merger

The Astrophysical Journal · 2017

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 {{s}} with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg<SUP>2</SUP> at a luminosity distance of {40}<SUB>-8</SUB><SUP>+8</SUP> Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 {M}<SUB>☉ </SUB>. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 {{Mpc}}) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient's position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta. <P />Any correspondence should be addressed to .

Kinematics of Parsec-scale Jets of Gamma-Ray Blazars at 43 GHz within the VLBA-BU-BLAZAR Program

The Astrophysical Journal · 2017

We analyze the parsec-scale jet kinematics from 2007 June to 2013 January of a sample of γ-ray bright blazars monitored roughly monthly with the Very Long Baseline Array at 43 GHz. In a total of 1929 images, we measure apparent speeds of 252 emission knots in 21 quasars, 12 BL Lacertae objects (BLLacs), and 3 radio galaxies, ranging from 0.02c to 78c; 21% of the knots are quasi-stationary. Approximately one-third of the moving knots execute non-ballistic motions, with the quasars exhibiting acceleration along the jet within 5 pc (projected) of the core, and knots in BLLacs tending to decelerate near the core. Using the apparent speeds of the components and the timescales of variability from their light curves, we derive the physical parameters of 120 superluminal knots, including variability Doppler factors, Lorentz factors, and viewing angles. We estimate the half-opening angle of each jet based on the projected opening angle and scatter of intrinsic viewing angles of knots. We determine characteristic values of the physical parameters for each jet and active galactic nucleus class based on the range of values obtained for individual features. We calculate the intrinsic brightness temperatures of the cores, {T}<SUB>{{b</SUB>},{int}}<SUP>{core</SUP>}, at all epochs, finding that the radio galaxies usually maintain equipartition conditions in the cores, while ∼30% of {T}<SUB>{{b</SUB>},{int}}<SUP>{core</SUP>} measurements in the quasars and BLLacs deviate from equipartition values by a factor &gt;10. This probably occurs during transient events connected with active states. In the Appendix, we briefly describe the behavior of each blazar during the period analyzed.

Teraelectronvolt pulsed emission from the Crab Pulsar detected by MAGIC

Astronomy and Astrophysics · 2016

<BR /> Aims: We investigate the extension of the very high-energy spectral tail of the Crab Pulsar at energies above 400 GeV. <BR /> Methods: We analyzed ~320 h of good-quality Crab data obtained with the MAGIC telescope from February 2007 to April 2014. <BR /> Results: We report the most energetic pulsed emission ever detected from the Crab Pulsar reaching up to 1.5 TeV. The pulse profile shows two narrow peaks synchronized with those measured in the GeV energy range. The spectra of the two peaks follow two different power-law functions from 70 GeV up to 1.5 TeV and connect smoothly with the spectra measured above 10 GeV by the Large Area Telescope (LAT) on board the Fermi satellite. When making a joint fit of the LAT and MAGIC data above 10 GeV the photon indices of the spectra differ by 0.5 ± 0.1. <BR /> Conclusions: Using data from the MAGIC telescopes we measured the most energetic pulsed photons from a pulsar to date. Such TeV pulsed photons require a parent population of electrons with a Lorentz factor of at least 5 × 10<SUP>6</SUP>. These results strongly suggest IC scattering off low-energy photons as the emission mechanism and a gamma-ray production region in the vicinity of the light cylinder.

The 2009 multiwavelength campaign on Mrk 421: Variability and correlation studies

Astronomy and Astrophysics · 2015

<BR /> Aims: We perform an extensive characterization of the broadband emission of Mrk 421, as well as its temporal evolution, during the non-flaring (low) state. The high brightness and nearby location (z = 0.031) of Mrk 421 make it an excellent laboratory to study blazar emission. The goal is to learn about the physical processes responsible for the typical emission of Mrk 421, which might also be extended to other blazars that are located farther away and hence are more difficult to study. <BR /> Methods: We performed a 4.5-month multi-instrument campaign on Mrk 421 between January 2009 and June 2009, which included VLBA, F-GAMMA, GASP-WEBT, Swift, RXTE, Fermi-LAT, MAGIC, and Whipple, among other instruments and collaborations. This extensive radio to very-high-energy (VHE; E&gt; 100 GeV) γ-ray dataset provides excellent temporal and energy coverage, which allows detailed studies of the evolution of the broadband spectral energy distribution. <BR /> Results: Mrk421 was found in its typical (non-flaring) activity state, with a VHE flux of about half that of the Crab Nebula, yet the light curves show significant variability at all wavelengths, the highest variability being in the X-rays. We determined the power spectral densities (PSD) at most wavelengths and found that all PSDs can be described by power-laws without a break, and with indices consistent with pink/red-noise behavior. We observed a harder-when-brighter behavior in the X-ray spectra and measured a positive correlation between VHE and X-ray fluxes with zero time lag. Such characteristics have been reported many times during flaring activity, but here they are reported for the first time in the non-flaring state. We also observed an overall anti-correlation between optical/UV and X-rays extending over the duration of the campaign. <BR /> Conclusions: The harder-when-brighter behavior in the X-ray spectra and the measured positive X-ray/VHE correlation during the 2009 multi-wavelength campaign suggests that the physical processes dominating the emission during non-flaring states have similarities with those occurring during flaring activity. In particular, this observation supports leptonic scenarios as being responsible for the emission of Mrk 421 during non-flaring activity. Such a temporally extended X-ray/VHE correlation is not driven by any single flaring event, and hence is difficult to explain within the standard hadronic scenarios. The highest variability is observed in the X-ray band, which, within the one-zone synchrotron self-Compton scenario, indicates that the electron energy distribution is most variable at the highest energies. <P />Appendix A is available in electronic form at <A href="http://www.aanda.org/10.1051/0004-6361/201424216/olm">http://www.aanda.org</A>The complete data set shown in Fig. 1 is only available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/576/A126">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/576/A126</A>

The Structure and Emission Model of the Relativistic Jet in the Quasar 3C 279 Inferred from Radio to High-energy γ-Ray Observations in 2008-2010

The Astrophysical Journal · 2012

We present time-resolved broadband observations of the quasar 3C 279 obtained from multi-wavelength campaigns conducted during the first two years of the Fermi Gamma-ray Space Telescope mission. While investigating the previously reported γ-ray/optical flare accompanied by a change in optical polarization, we found that the optical emission appears to be delayed with respect to the γ-ray emission by about 10 days. X-ray observations reveal a pair of "isolated" flares separated by ~90 days, with only weak γ-ray/optical counterparts. The spectral structure measured by Spitzer reveals a synchrotron component peaking in the mid-infrared band with a sharp break at the far-infrared band during the γ-ray flare, while the peak appears in the millimeter (mm)/submillimeter (sub-mm) band in the low state. Selected spectral energy distributions are fitted with leptonic models including Comptonization of external radiation produced in a dusty torus or the broad-line region. Adopting the interpretation of the polarization swing involving propagation of the emitting region along a curved trajectory, we can explain the evolution of the broadband spectra during the γ-ray flaring event by a shift of its location from ~1 pc to ~4 pc from the central black hole. On the other hand, if the γ-ray flare is generated instead at sub-pc distance from the central black hole, the far-infrared break can be explained by synchrotron self-absorption. We also model the low spectral state, dominated by the mm/sub-mm peaking synchrotron component, and suggest that the corresponding inverse-Compton component explains the steady X-ray emission.

The Spectral Energy Distribution of Fermi Bright Blazars

The Astrophysical Journal · 2010

We have conducted a detailed investigation of the broadband spectral properties of the γ-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately estimated Fermi γ-ray spectra with Swift, radio, infra-red, optical, and other hard X-ray/γ-ray data, collected within 3 months of the LBAS data taking period, we were able to assemble high-quality and quasi-simultaneous spectral energy distributions (SED) for 48 LBAS blazars. The SED of these γ-ray sources is similar to that of blazars discovered at other wavelengths, clearly showing, in the usual log ν-log ν F <SUB>ν</SUB> representation, the typical broadband spectral signatures normally attributed to a combination of low-energy synchrotron radiation followed by inverse Compton emission of one or more components. We have used these SED to characterize the peak intensity of both the low- and the high-energy components. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broadband colors (i.e., the radio to optical, α<SUB>ro</SUB>, and optical to X-ray, α<SUB>ox</SUB>, spectral slopes) and from the γ-ray spectral index. Our data show that the synchrotron peak frequency (ν<SUP> S </SUP> <SUB>peak</SUB>) is positioned between 10<SUP>12.5</SUP> and 10<SUP>14.5</SUP> Hz in broad-lined flat spectrum radio quasars (FSRQs) and between 10<SUP>13</SUP> and 10<SUP>17</SUP> Hz in featureless BL Lacertae objects. We find that the γ-ray spectral slope is strongly correlated with the synchrotron peak energy and with the X-ray spectral index, as expected at first order in synchrotron-inverse Compton scenarios. However, simple homogeneous, one-zone, synchrotron self-Compton (SSC) models cannot explain most of our SED, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. More complex models involving external Compton radiation or multiple SSC components are required to reproduce the overall SED and the observed spectral variability. While more than 50% of known radio bright high energy peaked (HBL) BL Lacs are detected in the LBAS sample, only less than 13% of known bright FSRQs and LBL BL Lacs are included. This suggests that the latter sources, as a class, may be much fainter γ-ray emitters than LBAS blazars, and could in fact radiate close to the expectations of simple SSC models. We categorized all our sources according to a new physical classification scheme based on the generally accepted paradigm for Active Galactic Nuclei and on the results of this SED study. Since the LAT detector is more sensitive to flat spectrum γ-ray sources, the correlation between ν<SUP> S </SUP> <SUB>peak</SUB> and γ-ray spectral index strongly favors the detection of high energy peaked blazars, thus explaining the Fermi overabundance of this type of sources compared to radio and EGRET samples. This selection effect is similar to that experienced in the soft X-ray band where HBL BL Lacs are the dominant type of blazars.

Flaring Behavior of the Quasar 3C 454.3 Across the Electromagnetic Spectrum

The Astrophysical Journal · 2010

We analyze the behavior of the parsec-scale jet of the quasar 3C 454.3 during pronounced flaring in 2005-2008. Three major disturbances propagated down the jet along different trajectories with Lorentz factors Γ &gt; 10. The disturbances show a clear connection with millimeter-wave outbursts, in 2005 May/June, 2007 July, and 2007 December. High-amplitude optical events in the R-band light curve precede peaks of the millimeter-wave outbursts by 15-50 days. Each optical outburst is accompanied by an increase in X-ray activity. We associate the optical outbursts with propagation of the superluminal knots and derive the location of sites of energy dissipation in the form of radiation. The most prominent and long lasting of these, in 2005 May, occurred closer to the black hole, while the outbursts with a shorter duration in 2005 autumn and in 2007 might be connected with the passage of a disturbance through the millimeter-wave core of the jet. The optical outbursts, which coincide with the passage of superluminal radio knots through the core, are accompanied by systematic rotation of the position angle of optical linear polarization. Such rotation appears to be a common feature during the early stages of flares in blazars. We find correlations between optical variations and those at X-ray and γ-ray energies. We conclude that the emergence of a superluminal knot from the core yields a series of optical and high-energy outbursts, and that the millimeter-wave core lies at the end of the jet's acceleration and collimation zone. We infer that the X-ray emission is produced via inverse Compton scattering by relativistic electrons of photons both from within the jet (synchrotron self-Compton) and external to the jet (external Compton, or EC); which one dominates depends on the physical parameters of the jet. A broken power-law model of the γ-ray spectrum reflects a steepening of the synchrotron emission spectrum from near-IR to soft UV wavelengths. We propose that the γ-ray emission is dominated by the EC mechanism, with the sheath of the jet supplying seed photons for γ-ray events that occur near the millimeter-wave core.

A change in the optical polarization associated with a γ-ray flare in the blazar 3C279

Nature · 2010

It is widely accepted that strong and variable radiation detected over all accessible energy bands in a number of active galaxies arises from a relativistic, Doppler-boosted jet pointing close to our line of sight. The size of the emitting zone and the location of this region relative to the central supermassive black hole are, however, poorly known, with estimates ranging from light-hours to a light-year or more. Here we report the coincidence of a gamma (γ)-ray flare with a dramatic change of optical polarization angle. This provides evidence for co-spatiality of optical and γ-ray emission regions and indicates a highly ordered jet magnetic field. The results also require a non-axisymmetric structure of the emission zone, implying a curved trajectory for the emitting material within the jet, with the dissipation region located at a considerable distance from the black hole, at about 10<SUP>5</SUP> gravitational radii.

Probing the Inner Jet of the Quasar PKS 1510-089 with Multi-Waveband Monitoring During Strong Gamma-Ray Activity

The Astrophysical Journal · 2010

We present results from monitoring the multi-waveband flux, linear polarization, and parsec-scale structure of the quasar PKS 1510 - 089, concentrating on eight major γ-ray flares that occurred during the interval 2009.0-2009.5. The γ-ray peaks were essentially simultaneous with maxima at optical wavelengths, although the flux ratio of the two wave bands varied by an order of magnitude. The optical polarization vector rotated by 720° during a five-day period encompassing six of these flares. This culminated in a very bright, ~1 day, optical and γ-ray flare as a bright knot of emission passed through the highest-intensity, stationary feature (the "core") seen in 43 GHz Very Long Baseline Array images. The knot continued to propagate down the jet at an apparent speed of 22c and emit strongly at γ-ray energies as a months-long X-ray/radio outburst intensified. We interpret these events as the result of the knot following a spiral path through a mainly toroidal magnetic field pattern in the acceleration and collimation zone of the jet, after which it passes through a standing shock in the 43 GHz core and then continues downstream. In this picture, the rapid γ-ray flares result from scattering of infrared seed photons from a relatively slow sheath of the jet as well as from optical synchrotron radiation in the faster spine. The 2006-2009.7 radio and X-ray flux variations are correlated at very high significance; we conclude that the X-rays are mainly from inverse Compton scattering of infrared seed photons by 20-40 MeV electrons.

Multiwavelength (Radio, X-Ray, and γ-Ray) Observations of the γ-Ray Binary LS I +61 303

The Astrophysical Journal · 2008

We present the results of the first multiwavelength observing campaign on the high-mass X-ray binary LS I +61 303, comprising observations at the TeV regime with the MAGIC telescope, along with X-ray observations with Chandra, and radio interferometric observations with the MERLIN, EVN, and VLBA arrays, in 2006 October and November. From our MERLIN observations, we can exclude the existence of large-scale (~100 mas) persistent radio jets. Our 5.0 GHz VLBA observations display morphological similarities to previous 8.4 GHz VLBA observations carried out at the same orbital phase, suggesting a high level of periodicity and stability in the processes behind the radio emission. This makes it unlikely that variability of the radio emission is due to the interaction of an outflow with variable wind clumps. If the radio emission is produced by a milliarcsecond scale jet, it should also show a stable, periodic behavior. It is then difficult to reconcile the absence of a large-scale jet (~100 mas) in our observations with the evidence of a persistent relativistic jet reported previously. We find a possible hint of temporal correlation between the X-ray and TeV emissions and evidence for radio/TeV noncorrelation, which points to the existence of one population of particles producing the radio emission and a different one producing the X-ray and TeV emissions. Finally, we present a quasi-simultaneous energy spectrum including radio, X-ray, and TeV bands. <P />Based on observations made with the MAGIC telescope, the Chandra X-ray Observatory, and the MERLIN, EVN, and NRAO VLBA arrays.

First Bounds on the High-Energy Emission from Isolated Wolf-Rayet Binary Systems

The Astrophysical Journal · 2008

High-energy gamma-ray emission is theoretically expected to arise in tight binary star systems (with high mass loss and high-velocity winds), although the evidence of this relationship has proven to be elusive so far. Here we present the first bounds on this putative emission from isolated Wolf-Rayet (WR) star binaries, WR 147 and WR 146, obtained from observations with the MAGIC telescope.

Upper Limit for γ-Ray Emission above 140 GeV from the Dwarf Spheroidal Galaxy Draco

The Astrophysical Journal · 2008

The nearby dwarf spheroidal galaxy Draco, with its high mass to light ratio, is one of the most auspicious targets for indirect dark matter (DM) searches. Annihilation of hypothetical DM particles can result in high-energy γ-rays, e.g., from neutralino annihilation in the supersymmetric framework. A search for a possible DM signal originating from Draco was performed with the MAGIC telescope during 2007. Analysis of the data results in a flux upper limit (2 σ) of 1.1 × 10<SUP>-11</SUP> photons cm<SUP>-2</SUP> s<SUP>-1</SUP> for photon energies above 140 GeV, assuming a pointlike source. A comparison with predictions from supersymmetric models is also given. While our results do not constrain the mSUGRA phase parameter space, a very high flux enhancement can be ruled out.

Constraints on the Steady and Pulsed Very High Energy Gamma-Ray Emission from Observations of PSR B1951+32/CTB 80 with the MAGIC Telescope

The Astrophysical Journal · 2007

We report on very high energy γ-ray observations with the MAGIC Telescope of the pulsar PSR B1951+32 and its associated nebula, CTB 80. Our data constrain the cutoff energy of the pulsar to be less than 32 GeV, assuming the pulsed γ-ray emission to be exponentially cut off. In the case that the cutoff follows a superexponential behavior, the cutoff energy can be as high as ~60 GeV. The upper limit on the flux of pulsed γ-ray emission above 75 GeV is 4.3×10<SUP>-11</SUP> photons cm<SUP>-2</SUP> s<SUP>-1</SUP>, and the upper limit on the flux of steady emission above 140 GeV is 1.5×10<SUP>-11</SUP> photons cm<SUP>-2</SUP> s<SUP>-1</SUP>. We discuss our results in the framework of recent model predictions and other studies.