Arnau Aguasca-Cabot

Context. Persistent jets from X-ray binaries that are aligned very close to the line of sight could be considered Galactic equivalents of blazars, or ‘microblazars’. They are also expected to power gamma-ray sources. Aims. We intend to assess a serious candidate apparently fulfilling many of the requirements to be considered a genuine member of this class: IRAS 18293−0941. Methods. We carried out an intense, multi-wavelength observational and theoretical study on our proposed candidate source. Results. With photometric and spectroscopic properties typical of a binary star, this system exhibits clear collimated and one-sided radio emission matching the effects of relativistic motion along a reduced ejection angle. Only fast variability is not observed; it is possibly smoothed by a dense circumstellar envelope. A physical scenario is consistently modelled that also gives credibility to its likely connection with LHAASO J1831−1007u ^* , an ultra-high-energy source in its immediate vicinity. Conclusions. Our reported identification not only helps fill a gap in Galactic taxonomy, it also potentially strengthens the role of the microblazar and microquasar families in our understanding of the most energetic Milky Way phenomena.

Aims . 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. Methods . 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 reanalyzed 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. Results . We did not detect any statistically significant γ-ray emission from NGC 4278 with MAGIC. The corresponding upper limits obtained using the entire MAGIC dataset ( F _{UL,>150 GeV} = 1.5 × 10 ⁻¹² 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, on the order of L _jet ∼ 10 ⁴² 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.

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, the Swift-X-Ray Telescope and the Einstein Probe Follow-up X-ray Telescope. 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.

Flat spectrum radio quasars (FSRQs) often remain undetected at very-high-energy (VHE, E > 100 GeV) γ rays due to their typically soft spectra and large distance. In December 2023, the Large-Sized Telescope prototype (LST-1) detected for the first time VHE γ-ray emission from the FSRQ OP 313 during an extraordinarily bright flare, becoming the furthest blazar ever observed at VHE with z=0.997. We aim to characterize the γ-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 gackground light (EBL), with the goal of evaluating the attenuation of VHE γ-ray photons. Thanks to an intensive multiwavelength campaign, we also study in detail the broadband emission of the source. We characterize the γ-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 -LAT observations. Fermi This dataset also enables us to evaluate the EBL attenuation by systematically exploring the EBL intensity over the γ-ray spectrum. . Finally, we study the multiwavelength emission and interpret the broadband spectral energy distribution (SED) within blazar radiative models including the thermal contributions from the accretion disk, dusty torus and broad line region. We measured significant VHE γ-ray emission during the December 2023 flare at a level of 0.3 Crab units above 100 GeV . We also characterize the flare brightness in the high-energy (HE, E>100 MeV) γ-ray band, which was found to be a factor of 50 above the average emission seen by -LAT. The HE and VHE observations allow us to set constrains on the EBL density, resulting in an upper limit at 0.6 μm of łambda I_ Fermi łambda <8.74 nW m 2 $ sr^-1 with LST-1 data, and łambda I_ łambda <14.7 nW m^2 sr^-1 with MAGIC data, including systematic uncertainties. Finally, thanks to the extensive multiwavelength 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 γ-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 $γ-ray emission observed .

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.

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 > 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.

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 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 < 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.

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 > 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.