Elisabete Maria de Gouveia Dal Pino

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.

In this work, we numerically investigate a crucial parameter in turbulence-induced magnetic reconnection theory: the particle acceleration time. Employing simulations of magnetically dominated turbulent relativistic jets, we examine particles accelerated either during the jet’s evolution or in a nearly stationary postprocessed state. We derive acceleration times and compare them with theoretical predictions for both the Fermi and drift regimes identified in the simulations. In the Fermi regime, theory predicts that acceleration time remains nearly constant with particle energy for a fixed reconnection velocity, as energy grows exponentially in time. Conversely, we expect the reconnection acceleration time to depend on the current sheet’s thickness and the reconnection velocity, a dependence recently revisited by S. Xu & A. Lazarian. We test their relations using statistical distributions of current sheet thicknesses and reconnection speeds and find that the average acceleration time agrees well with their predictions, showing a weak dependence on particle energy. We further validate these findings by comparing them with in situ acceleration times from 50,000 test particles. The results show good agreement, particularly for the fastest reconnection acceleration conditions. When simulating longer acceleration periods in a quasi-steady turbulent jet snapshot, the acceleration time during the Fermi regime remains nearly constant up to a critical energy—where the particles’ Larmor radius matches the largest current sheet thickness. Above this threshold, particles enter the slower drift regime, where acceleration time becomes strongly energy dependent, as expected. Overall, our results confirm the theoretical framework and highlight the dominance of the Fermi acceleration process up to very high energies.

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.

We investigate the development of tearing-mode instability using the highest-resolution 2D magnetohydrodynamic simulations of reconnecting current sheets performed on a uniform grid, for Lundquist numbers of 10 ³ ≤ S ≤ 5 × 10 ⁵ , reaching up to 65,536 ² grid cells. We demonstrate a Sweet–Parker scaling of the reconnection rate V _rec ∼ S ^−1/2 up to Lundquist numbers S ∼ 10 ⁴ . For larger values of Lundquist number, between 2 × 10 ⁴ ≤ S ≤ 2 × 10 ⁵ , plasmoid formation sets in, leading to a slight enhancement of the reconnection rate, V _rec ∼ S ^−1/3 , consistent with the prediction from linear-tearing-mode-induced reconnection, indicating that reconnection remains resistivity-dependent and therefore slow. In this range of S -values, the plasmoids do not undergo a merger cascade, as they are rapidly advected out of the reconnection layer. Only for S > 2 × 10 ⁵ , we observe the nonlinear development of the tearing-mode instability, with plasmoid coalescence and a saturation of the reconnection rate at V _rec / V _A ∼ 0.01. At such high S , however, the corresponding Reynolds number is large, reaching Re > 2000 even on scales comparable to the current-sheet thickness. We therefore conclude that, in astrophysical systems, it is essential to account for the dominant influence of turbulence and 3D effects in the reconnection process.

High-energy astrophysical sources such as active galactic nuclei, quasars, X-ray binaries, and gamma-ray bursts are powered by mechanisms that convert gravitational or rotational energy into radiation, jets, and relativistic outflows. Understanding the physics of these processes remains a major challenge. Black holes have traditionally served as the central engines behind such phenomena, with well established energy extraction mechanisms including the Penrose process, the Blandford-Znajek process, and the Banados-Silk-West mechanism. However, studies in general relativity indicate that, under certain conditions, gravitational collapse may lead to the formation of naked singularities or other horizonless compact objects, which could in principle allow more efficient energy extraction than classical black holes. This brief review summarizes recent progress on energy extraction mechanisms in naked singularity spacetimes. We examine the roles of rotation, electromagnetic fields, and particle interactions in shaping extraction efficiency and dynamics. Particular attention is given to negative energy orbits and ergoregion physics, which enable Penrose type and magnetic Penrose mechanisms without an event horizon. We also discuss collisional Penrose processes and particle acceleration near the singularity, emphasizing their potential astrophysical implications. By comparing extraction efficiencies and physical conditions in black holes and naked singularities, we highlight how the absence of a horizon fundamentally alters the dynamics of energy release. These results suggest that naked singularities may serve as natural laboratories for strong field gravity and as alternative engines for high-energy astrophysical phenomena in the era of multi-messenger observations.

A compact object illuminated by background radiation produces a dark silhouette. The edge of the silhouette or shadow (alternatively, the apparent boundary or the critical curve) is commonly determined by the presence of the photon sphere (or photon shell in the case of rotating spacetime), corresponding to the maximum of the effective potential for null geodesics. While this statement stands true for Kerr black holes, here we remark that the apparent boundary (as defined by Bardeen) forms under a more general condition. We demonstrate that a shadow forms if the effective potential of null geodesics has a positive finite upper bound and includes a region where photons are trapped or scattered. Our framework extends beyond conventional solutions, including but not limited to naked singularities. Furthermore, we clarify the difference between the apparent boundary of a dark shadow and the bright ring on the screen of a distant observer. These results provide a unified theoretical basis for interpreting observations from the Event Horizon Telescope (EHT) and guiding future efforts towards extreme-resolution observations of compact objects.

A compact object illuminated by background radiation produces a dark silhouette. The edge of the silhouette or shadow (alternatively, the apparent boundary or the critical curve) is commonly determined by the presence of the photon sphere (or photon shell in the case of rotating spacetime), corresponding to the maximum of the effective potential for null geodesics. While this statement stands true for Kerr black holes, here we remark that the apparent boundary (as defined by Bardeen) forms under a more general condition. We demonstrate that a shadow forms if the effective potential of null geodesics has a positive finite upper bound and includes a region where photons are trapped or scattered. Our framework extends beyond conventional solutions, including but not limited to naked singularities. Furthermore, we clarify the difference between the apparent boundary of a dark shadow and the bright ring on the screen of a distant observer. These results provide a unified theoretical basis for interpreting observations from the Event Horizon Telescope (EHT) and guiding future efforts towards extreme-resolution observations of compact objects.