Understanding the physical conditions in the vicinity of black holes is one of the major challenges of highenergy

astrophysics. The question of how supermassive black holes (SMBHs) at the center of galaxies form,

grow and accrete matter will largely remain open during the next decade. In the early 2030s, the launch of the

next two large-class missions of the European Space Agency (ESA), ATHENA and LISA, will be a game-changer

for how we study SMBHs. While LISA is designed to observe the low-frequency gravitational-wave signal

Resumé

Since 2013 and the first observation of high-energy astrophysical neutrinos in the IceCube Neutrino Observatory, neutrinos constitute a new messenger to study the extreme Universe, and large neutrino telescopes have been working towards the identification of sources.  While recent multi-messenger observations suggest that blazars may be the first identifiable sources of this observed neutrino flux, other source populations emitting neutrinos remain unidentified. Among promising candidates are short gamma-ray bursts (SGRBs) resulting from the merger of two neutron stars.