It is now well established that active galactic nuclei (AGN) are the dominant class of extragalactic sources in the gamma-ray sky. Among them, blazars—AGN with jets oriented toward the observer—constitute the majority. In contrast, misaligned AGN, or radio galaxies, account for only about 2% of the 4LAC-DR2 catalog of AGN detected by Fermi in the gamma-ray band. At TeV energies, only six radio...
PKS 2155-304 is a high-frequency peaked BL Lac object (HBL) located at z = 0.116. In 2006, it experienced an extreme very-high-energy (VHE) flare observed by H.E.S.S., which revealed minute-scale variability and posed challenges for standard emission models. Since then, it has become a key target for probing Lorentz Invariance Violation (LIV) and extragalactic background light (EBL)...
The intermediate synchrotron-peaked BL Lac B2 1811+31 (z=0.117) underwent a period of high activity from the optical to the very high-energy (VHE; 100 GeV < E < 100 TeV) gamma ray band in 2020. Following a high-state detection by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope in the high-energy gamma-ray band (HE; 100 MeV < E < 100 GeV), a dedicated multiwavelength...
The Seyfert galaxies NGC 1068 and NGC 4151 have emerged as the most promising counterparts of 4.2σ and 3.0σ neutrino excesses detected by IceCube in the TeV energy range.
Gamma rays and neutrinos are co-produced at the same flux level via hadronic interactions between the parent proton population and the ambient matter and radiation in the neutrino-emitting region.Observations of NGC 1068...
A power-law distribution with a high energy cut-off can typically describe the gamma-ray energy spectrum for Active Galactic Nuclei (AGNs). The Fermi Large Area Telescope (LAT) can probe many of these objects for which this cut-off sits in the GeV domain. However, measurement of the spectral energy cutoff requires large photon statistics, which are not always available because of Fermi-LAT's...
Fermi-LAT is one of the major gamma-ray observatories in the hundreds of MeV to few TeV energies. In this work, we searched for gamma-ray emission from Jupiter using 15 years of Fermi-LAT data in the energy range from 100 MeV to 2 TeV.
Jupiter’s path on the sky was partitioned into 1202 steps, each one corresponding to a 0.5 deg displacement and a likelihood analysis was performed at each...
Dark Matter (DM) remains a great mystery in modern physics. Among various candidates, the weakly interacting massive particles (WIMPs) scenario stands out and is under extensive study. The detection of the hypothetical gamma-ray emission from WIMP annihilation could act as a direct probe of electroweak-scale interactions, complementing DM collider searches and other direct DM detection...
Supernova remnants (SNRs) are key cosmic ray (CR) sources, as strongly supported by extensive γ-ray observations of the characteristic pion bump, particularly by AGILE-GRID and Fermi-LAT, which confirmed them as hadronic accelerators. LHAASO recently detected >100 TeV γ-rays from several Galactic sources, including some regions containing SNRs. However, current theoretical models suggest that...
In recent years, Young massive stellar clusters (YMSCs) have been identified as Galactic CR factories. In clusters younger than 3 Myr, stellar winds are expected to drive CR acceleration through different mechanisms, such as acceleration at the collective cluster wind termination shocks (WTS). Understanding the contribution of YMSC and stellar winds to the production of Galactic CRs is...
Pseudo-scalar particles, like QCD axions and Axion-like-Particles (ALPs), emerge in many extension of the Standard Model and have been recognized to be among the best Dark Matter candidates. Even if very weakly-interacting, ALPs can be copiously in the core of massive stars at the end of their life. In this regard, Core-Collapse Supernovae (SNe) are expected to be powerful sources of novel...
Axion-like particles (ALPs) can be copiously produced in binary neutron star (BNS) mergers via nucleon-nucleon bremsstrahlung, provided the ALP-nucleon coupling $g_{aN}$ is sufficiently large. If ALPs also couple to photons through the coupling $g_{a\gamma}$, they can convert into gamma rays in the presence of strong magnetic fields, such as those surrounding the merger remnant and in the...
Axion-like particles (ALPs) are a common prediction of several extensions of the Standard Model and could be detected through their coupling to photons, which enables ALP-photon conversions in external magnetic fields. This conversion could lead to two distinct signatures in gamma-ray spectra of blazars: a superimposition of energy-dependent "wiggles" on the spectral shape, and a hardening at...
Axion-like particles (ALPs) are theoretical particles proposed in several extensions of the Standard Model of elementary particles. Aside from being viable dark matter candidates, their predicted interaction with photons in external magnetic fields makes them particularly relevant in observations of very-high-energy (VHE) gamma-ray sources. Specifically, this interaction is expected to leave...
Gamma-ray observations of astrophysical neutrino sources are crucial for understanding neutrino production in extreme cosmic environments. The Cherenkov Telescope Array Observatory (CTAO), the first open-access ground-based gamma-ray observatory, is under construction in both hemispheres, CTAO-North in La Palma (Spain) and CTAO-South in the Atacama Desert (Chile). Covering an energy range from...
Supernovae (SNe) are explosive phenomena known to emit across the entire electromagnetic spectrum, up to soft gamma-rays. Very high energy (VHE) gamma-rays are expected to be produced through shock interactions between dense circumstellar material and the supernova ejecta. However no clear detection has been achieved so far by past or current generation of imaging atmospheric Cherenkov...
The event GW170817, which was for the first time associated to the e.m. event GRB 170817A, proved that at least a portion of Gamma-Ray Bursts (GRBs) are counterparts of Gravitational Wave (GW) events involving a neutron star. Despite the incredible amount of follow up detections of GW170817, though, no other event of this kind has been later observed. Of great importance is to set up a joint...
The early X-ray afterglows of Gamma-Ray Bursts (GRBs), observed with the Swift X-ray Telescope (XRT; 0.3–10 keV) onboard the Neil Gehrels Swift Observatory, have revealed distinct temporal features beyond those predicted by the standard forward shock afterglow model. Components in the XRT light curve, such as steep decay, flares, and plateaus, suggest more complex afterglow physics. These...
The Fermi mission is a space-based observatory designed to study the gamma-ray sky. It consists of two main instruments: the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM), covering a broad energy range from ~10 keV to >300 GeV. Among its core scientific goals is the detection and characterization of Gamma-ray Bursts (GRBs), although the mechanisms driving their prompt...
