Stefano Vercellone

The ASTRI-Horn telescope is a prototype Imaging Atmospheric Cherenkov Telescope (IACT) developed within the Astrofisica con Specchi a Tecnologia Replicante Italiana (ASTRI) project. It represents the first implementation of a dual-mirror Schwarzschild–Couder (SC) optical configuration for ground-based gamma-ray IACT observations. It also served as a testbed for the Small-Sized Telescopes (SSTs) of the Cherenkov Telescope Array Observatory (CTAO) that will host at least 37 SSTs at the Paranal site in Chile, and for the forthcoming ASTRI Mini-Array. Installed in 2014 at the INAF “M.C. Fracastoro” observing station on Mount Etna, the telescope is equipped with a SiPM-based Cherenkov camera and a complete control and data acquisition system. Between 2020 and 2022, after the demonstration of the validity of the SC configuration in observing gamma-ray sources, ASTRI-Horn underwent refurbishment activities that included mirror replacement, new optical coatings, camera upgrades, and software improvements. The telescope has since been re-validated through calibration campaigns and observational tests, including the detection and spectral characterization of the Crab Nebula at TeV energies. The results confirm the correct operation of the optical system, camera electronics, and data processing chain, providing a consolidated basis for the design and development of the ASTRI Mini-Array and future CTAO-SST telescopes.

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 .

Context. On April 14, 2009, the AGILE satellite detected a γ -ray flare from an unknown transient source. Subsequent X-ray follow-up observations with Swift and INTEGRAL identified the supergiant fast X-ray transient (SFXT) IGR J17354−3255 as the best candidate counterpart, based on positional coincidence and a similar temporal behaviour. Aside from this hint, no SFXT has been firmly detected at high energies to date. Overall, SFXTs comprise a subclass of high-mass X-ray binaries (HMXBs) that host a massive OB supergiant star as a companion donor. They tend to display the most extreme X-ray variability among HMXBs. These systems might be able to emit photons at MeV-TeV energies in the form of fast flares lasting from hours to a few days, with a low-duty cycle. Aims. In this work, we analyse archival AGILE data to search for γ -ray flares consistent with IGR J17354−3255 and evaluate their possible physical origin. Methods. We identified a transient source, AGL J1736−3250, which emitted 19 γ -ray flares and was seen to be positionally consistent with IGR J17354−3255. Most flares, detected on a 1 d timescale, concentrate most of their emission on two, four, and six hour timescales, resembling those observed in the X-ray band from IGR J17354−3255. Results. An orbital phase analysis revealed that approximately half of the γ -ray activity occurs around the apastron passage of the compact object hosted in the binary system. We also incorporated archival Swift and INTEGRAL observations to provide phase-folded light curves at lower energies. Our collected results strongly support a physical association between IGR J17354−3255 and AGL J1736−3250, offering evidence that SFXTs could constitute a new class of Galactic high-energy transient emitters.

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. 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 ⁻³ . 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.

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_γ(> 100 GeV)≤3.61 × 10⁻¹² cm⁻² s⁻¹ 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 Ṁ/u_w ≤ 10⁻⁴ M_⊙/ yr s/km, although gamma-gamma absorption could potentially have skewed this estimation, effectively weakening our constraint. From spectro-photometric observations we found progenitor parameters of M_pr = 11 – 20 M_⊙ and R_pr = 531 ± 125 R_⊙. Finally, using archival images from the Hubble Space Telescope, we constrained the luminosity of the progenitor star to log (L_pr/L_⊙) ≤ 4.82 and its effective temperature to T_pr ≤ 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.

We explored the expected properties of the neutrino emission from accreting neutron stars in X-ray binaries using numerical simulations. The simulations are based on a model in which neutrinos are produced by the decay of charged pions and kaons, generated in inelastic collisions between protons accelerated up to TeV energies in the magnetosphere of a magnetized (B ∼ 10¹² G) neutron star and protons of the accretion disc. Our results show that this process can produce strong neutrino emission up to a few tens of TeV when the X-ray luminosity is above ∼10³⁹ erg s⁻¹, as in ultra-luminous X-ray (ULX) pulsars. We show that neutrinos from a transient Galactic ULX pulsar with L_x ≈ 5 × 10³⁹ erg s⁻¹ can be detected with kilometre-scale detectors such as IceCube if the source is within about 3–4 kpc. We also derived an upper limit on the neutrino flux from the Galactic ULX pulsar Swift J0243.6+6124 using IceCube data, a result that has not been previously reported. Our findings establish a new benchmark for future astrophysical neutrino observations, critical for interpreting data from current and upcoming instruments with significantly improved sensitivity.

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 10⁻¹¹ erg cm⁻² s⁻¹. Our late-time VHE observations can help disentangle the degeneracy among the models and provide valuable new insight into the structure of GRB jets.

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_stat)−0.6_sys^+0.2_sys, 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.