Supermassive black holes (SMBHs) located at the center of galaxies, particularly those powering luminous Active Galactic Nuclei (AGNs), and stellar-mass black holes in X-ray bright binary systems (XRBs), exhibit highly variable and often transient X-ray and gamma-ray emissions. Additionally, Tidal Disruption Events (TDEs) —which occur when a star approaches a massive black hole and is tidally disrupted— produce electromagnetic ares peaking in the UV and soft X-rays. 

The origin of cosmic-rays remains one of the most important open questions in astrophysics. A powerful tool to study their acceleration sites are multi-messenger (photons and neutrinos) observations: wherever a cosmic-ray (a proton or an atomic nucleus) is accelerated, it interacts with its environment producing pions that then decay into photons, neutrinos and electrons.  γ and ν can escape the region and travel on geodesics to Earth, pinpointing their source in the Universe.