arXiv subjects

A. Biland

Publications and source records attributed to A. Biland.

At least 19 recordsLinked to original sources

arXiv2026

Revisiting EBL Constraints from Gamma-Ray Observations: A Critical Assessment and Methodological Improvements

The extragalactic background light (EBL), ranging from the infrared to the ultraviolet bands, is the second most intense photon field in the universe, surpassed only by the cosmic microwave background (CMB). It is primarily generated by starlight in galaxies, either directly or through absorption by dust and re-emission at longer wavelengths. Very high energy (VHE, $E$ > $100\,$GeV) photons can be absorbed via $\gamma \gamma$ interactions with the EBL during their propagation across cosmological distances, providing an indirect method to probe the EBL by studying its impact on the energy spectra of distant VHE sources. This paper examines the robustness of EBL constraints derived from gamma-ray data, critically assessing the assumptions made in previous studies about the intrinsic source spectra, the uncertainties of the observations and the validity of the analysis tools. We find that earlier studies likely underestimated the uncertainties in the EBL intensity constraints, underscoring the need to account for systematic uncertainties comprehensively. By employing a Monte Carlo (MC) simulation and a plausible model for unknown systematic errors, we compute more realistic uncertainties. Additionally, we discuss possible alternatives to set EBL constraints, relaxing the assumptions on the intrinsic spectra of gamma-ray sources, with the goal of achieving more robust constraints.

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arXiv2026

Constraints on the VHE counterpart of two binary black hole mergers observed by the MAGIC and CTAO LST-1 telescopes

We present very-high-energy gamma-ray observations of two binary black hole merger candidates, GW240615_113620 and GW241125_010116, performed with the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes and the first Large-Sized Telescope of the Cherenkov Telescope Array Observatory's (CTAO LST-1). GW240615_113620 was the best localized event of the fourth observing run of the LIGO-Virgo-KAGRA gravitational waves interferometers. GW241125_010116 was temporally and spatially coincident with a sub-threshold short-duration burst detected with the Swift-Burst Alert Telescope (BAT), the Swift-X-Ray Telescope (XRT) and the Einstein Probe Follow-up X-ray Telescope (FXT). We observed the two events in stereoscopic mode, taking advantage of the improved sensitivity of joint MAGIC+LST-1 observations. No detection was achieved in the GeV-TeV gamma-ray band for any of the two sources. The unfavourable observing conditions of both events posed a challenge for a standard analysis and therefore a non standard analysis was necessary for both objects. Owing to the small localization area and the association with a GRB-like burst respectively, these events represented an unprecedented opportunity to study in details the electromagnetic emission from binary black holes merger events and, in particular, we discussed two theoretical models that predict a detectable gamma-ray emission and the possible future applications.

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arXiv2026

Detection of the distant quasar OP 313 with the first Large-Sized Telescope of CTAO

In December 2023, the Large-Sized Telescope prototype (LST-1) detected for the first time VHE $\gamma$-ray emission from the FSRQ OP~313 becoming the furthest blazar ever observed at VHE with z=0.997. We aim to characterize the $\gamma$-ray emission of OP 313 during this flare, comparing it with its average emission state in order to understand the processes leading to this detection. Its remarkable distance also enables studies on the Extragalactic Background Light (EBL), with the goal of evaluating the attenuation of VHE $\gamma$-ray photons. We characterize the $\gamma$-ray emission during the flare in December 2023 and the low emission state observed in January 2024 thanks to the LST-1 and MAGIC data and quasi-simultaneous \textit{Fermi}-LAT observations. This dataset also enables us to evaluate the EBL attenuation by systematically exploring the EBL intensity over the $\gamma$-ray spectrum. Finally, we study the multi-wavelength emission and interpret the broadband spectral energy distribution (SED) within blazar radiative models including the thermal contributions from the accretion disc, dusty torus and broad line region. We also characterize the flare brightness in the high-energy (HE, $E>100$~MeV) $\gamma$-ray band, that was found to be a factor 50 above the average emission seen by \textit{Fermi}-LAT. The HE and VHE observations allow us to set constrains to the EBL density. Finally, thanks to the extensive multi-wavelength campaign organized, we are able to construct and model the broadband SED of OP~313 within the framework of a two-zone leptonic model where the $\gamma$-ray emission is produced via inverse-Compton scattering of the broad line region, accretion disk and dusty torus photon fields. However the dominant external photon field remains unknown, as several combinations are able to successfully explain the $\gamma$-ray emission observed.

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arXiv2026

Long-term study of the gamma-ray emission of Cygnus X-3 with MAGIC and Fermi-LAT

Cygnus X-3 is a microquasar composed of a compact object of unknown nature closely orbiting around a Wolf-Rayet star. The particularities of this source make it a unique case among microquasars. This fact, together with its recent establishment as a PeV particle accelerator, makes Cygnus X-3 a very interesting target for the investigation of the physical processes leading to gamma-ray production. In this work, the TeV and GeV gamma-ray emission of Cygnus X-3 is studied in order to determine its origin and constrain the properties of the system. For that purpose, a point-like analysis of 130 h of data taken with the MAGIC telescopes between 2013 and 2024 was performed, which represents the largest available sample for Cygnus X-3 at $\sim$TeV energies. Additionally, contemporary data from Fermi-LAT were also analysed to better contextualize the MAGIC observations. For a more detailed investigation of the source physics, the data were divided into three subsets according to the flaring state of the source and orbital phase. No significant detection of Cygnus X-3 is found between 0.1 and 7 TeV for any of the datasets, and differential and integral flux upper limits are reported over the long-term monitoring of the source. The Fermi-LAT fluxes can be considered compatible with previous results, taking into account the different data samples used across studies. The MAGIC upper limits presented in this work represent the most constraining ones up to date at $\sim$TeV energies. An eventual detection of Cygnus X-3 at these energies would significantly constrain the source properties, and is not unreasonable to expect given that the source has already been detected in both the GeV and PeV regimes during flaring states. Further observations of Cygnus X-3 at energies above tens of GeV would be valuable for this purpose.

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arXiv2026

The January 2010 flare of Mrk421: Insights from a stochastic acceleration model

Mrk421 displayed its highest flux state ever observed in February of 2010 with very high TeV 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. We build up on the temporal variability and correlations studied in the previous work (MAGIC collaboration - Abe et al. 2025) and attempt to improve the SED model fits with a physics oriented approach. The multi-wavelength data was 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 fit to the SEDs and the resulting trends and phenomenology were characterised in context of theoretical literature. An expanding emission region model was also tested. We find the spectral variability to be well in agreement with stochastic acceleration. Our analysis suggests that the standard LPPL distribution develops a Maxwellian pile-up component at the transition from acceleration to cooling dominated phase on 3 nights in the dataset, as also hinted 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.

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arXiv2026

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

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 multi-messenger astronomy. The detection of TeV emission from GRBs motivates follow-up observations with the Cherenkov Telescope Array Observatory (CTAO), 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 multi-step 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.

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arXiv2026

Extreme Blazars Observed with MAGIC: Second Catalog Release

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 $10^{17}$ Hz in X-rays. Such emission is then reprocessed and produces radiation extending deeply into very-high-energy (VHE, energy E>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 hours 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<E<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.

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arXiv2026

MAGIC observations of NGC 4278. The first low-luminosity radio galaxy with compact jets detected at TeV energies

The Large High Altitude Air Shower Observatory (LHAASO) Collaboration has recently reported the first detection at TeV energies of a low-luminosity radio galaxy, NGC 4278. The aim of this work is to investigate the high-energy properties of NGC 4278 during the flaring and subsequent quasi-quiescent states with the Florian Goebel Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes. NGC 4278 is located in the field of view of two blazars, 1ES 1215+303 and 1ES 1218+304, previously observed by the MAGIC telescopes. Therefore, we re-analyzed MAGIC observations made between 2010 and 2024 on these sources. We also modeled the broadband spectral energy distribution of the source during and after the flaring state at TeV energies. We did not detect any statistically significant $\gamma$-ray emission from NGC 4278 with MAGIC. The corresponding upper limits obtained using the entire MAGIC dataset ($F_{{\rm UL, }\, >150\, \mathrm{GeV}}=1.5 \times 10^{-12}\, \mathrm{ph \, s^{-1}\, cm^{-2}}$) are consistent with the LHAASO results. The best-fit models obtained for both emission states suggest that the emitting region is strongly particle-dominated, and an efficient acceleration mechanism has to be in action in order to reach TeV energies. The transition between the flaring and quasi-quiescent state cannot be explained by a simple radiative cooling of the emitting particles. The inferred jet power, of the order of $L_{\rm jet}\sim 10^{42}\, \mathrm{erg\,s^{-1}}$, is dominated by the kinetic component in both states and it is in a good agreement with previous, time-averaged observational estimates, supporting the idea that such high-energy flares might be recurrent. The jet, however, remains too weak to break the host-galaxy confinement.

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arXiv2026

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

TON\,0599 (z=0.7247) belongs to the few flat spectrum radio quasars (FSRQs) detected in the very high energy (VHE, $E > 100$\,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 ($E > 100$\,MeV) band obtained from simultaneous observations with {\textit 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 (WEBT) Collaboration from November to March, as well as observations in X-ray and optical--UV bands with instruments on board the \textit{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 (EC) scattering of photons from the dusty torus.

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arXiv2025

Prompt Searches for Very-High-Energy {\gamma}-Ray Counterparts to IceCube Astrophysical Neutrino Alerts

The search for sources of high-energy astrophysical neutrinos can be significantly advanced through a multi-messenger approach, which seeks to detect the gamma rays that accompany neutrinos as they are produced at their sources. Multi-messenger 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 highenergy (HE, E > 1 GeV) and very-high-energy (VHE, E > 100 GeV) gamma 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: followup observations of single astrophysical neutrino candidate events (such as IceCube-170922A), and observation of known gamma-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 September 2017 (after the IceCube-170922A event) and January 2021. Our study found no associations between gamma-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 gamma-ray flux.

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arXiv2025

Time-Dependent Modeling of the Sub-Hour Spectral Evolution During the 2013 Outburst of Mrk 421

In April 2013, the TeV blazar Markarian~421 underwent one of its most powerful emission outbursts to date. An extensive multi-instrument campaign featuring MAGIC, VERITAS, and \textit{NuSTAR} provided comprehensive very-high-energy (VHE; $E > 100$\,GeV) and X-ray coverage over nine consecutive days. In this work, we perform a detailed spectral analysis of the X-ray and VHE emissions on sub-hour 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. 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 sub-hour 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 are consistent with a shock-acceleration scenario where the shock compression ratio evolves by a factor of $\sim2$. 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.

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arXiv2025

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

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 $\gamma$-ray range owing to their lack of astrophysical $\gamma$-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 ultra-faint 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 $\sim$10 TeV, reaching velocity-averaged annihilation cross sections of $\sim$5$\times$10$^{-25}$ cm$^3$ s$^{-1}$ and decay lifetimes up to $\sim$10$^{26}$ s for combined limits. The dominant uncertainties arise from the imprecise determination of the DM content, particularly for ultra-faint dSphs. Observation strategies are proposed that optimise either deep exposures of the best candidates or diversified target selections.

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arXiv2025

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

Seyfert galaxies are emerging as a promising source class of high-energy neutrinos. The Seyfert galaxies NGC 4151 and NGC 1068 have come up respectively as the most promising counterparts of a 3$\sigma$ and of a 4.2$\sigma$ 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 unveil the mechanism and site of neutrino production. In this work, we present the first results of the VHE observations ($\sim$29 hours) of NGC 4151 with the MAGIC telescopes. We detect no gamma-ray excess in the direction of NGC 4151, and we derive constraining upper limits on the VHE gamma-ray flux. The integral flux upper limit (at the 95% confidence level) above 200 GeV is $f = 2.3 \times 10^{-12}$ cm$^{-2}$ s$^{-1}$. The comparison of the MAGIC and IceCube measurements suggests the presence of a gamma-ray obscured accelerator, and it allows us to constrain the gamma-ray optical depth and the size of the neutrino production site.

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arXiv2025

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

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 > 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 < 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.

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arXiv2025

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

Among the blazars whose emission has been detected up to very-high-energy (VHE; 100 GeV < E < 100 TeV) gamma 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 emissions from radio to gamma-ray energies was achieved with quasi-simultaneous observations, which led to the first-time detection of VHE gamma-ray emission from the source with the MAGIC telescopes. In this work, we present a comprehensive multi-wavelength view of B2 1811+31 employing data from MAGIC, Fermi-LAT, Swift-XRT, Swift-UVOT and from several optical and radio ground-based telescopes. We investigate the variability, cross-correlations and classification of the source emissions during low and high states. 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 few hours in the high-energy (HE; 100 MeV < E < 100 GeV) gamma-ray band poses an upper limit of 6 x 10^{14} delta_D cm to the size of the emission region responsible for the gamma-ray flare, delta_D being the relativistic Doppler factor of the region. During the 2020 high state, the average spectrum became harder in the X-ray and HE gamma-ray bands compared to the low states. Conversely, during different activity periods, we find harder-when-brighter trends in X rays and a hint of softer-when-brighter trends at HE gamma rays. Gamma-optical correlation indicates the same emission regions dominate the radiative output in both ranges, whereas the levolution at 15 GHz shows no correlation with the flux at higher frequencies. We test one-zone and two-zone synchrotron-self-Compton models for describing the broad-band spectral energy distribution during the 2020 flare and investigate the self-consistency of the proposed scenario.

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arXiv2025

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

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. 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. 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$\,>\,$100$\,$GeV) gamma rays to radio with MAGIC, VERITAS, Fermi-LAT, RXTE, Swift, GASP-WEBT, VLBA, and a variety of additional optical and radio telescopes. We investigated the variability and correlation behavior among different energy bands in great detail. We find the strongest variability in X-rays and VHE gamma rays, and PSDs compatible with power-law functions. We observe strong correlations between X-rays and VHE gamma rays. We also report a marginally significant positive correlation between high-energy (HE; E$\,>\,$100$\,$MeV) gamma rays and the ultraviolet band. We detected marginally significant 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. The 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 radio features prevent us from firmly associating them to the specific flares recorded during the 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.

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arXiv2025

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

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 100MeV 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 100MeV 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.

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arXiv2024

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

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 < E < 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 multiwavelength 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 > 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, as observed by MAGIC and the Fermi-LAT telescopes, yielded a conservative 95\% confidence upper limit of z \leq 0.244 for the redshift of this blazar.

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