Cette thèse est proposée au sein du groupe d'Astrophysique des Hautes Energies (AHE) du laboratoire APC, qui est cœur du développement de l'astronomie multi-messagers des phénomènes transitoires extrêmes dans l'Univers.

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