Mergers of supermassive black holes in astrophysical environments

Modeling the late inspiral and mergers of supermassive black holes (BHs) is central to understanding accretion processes and the conditions under which electromagnetic (EM) emission accompanies gravitational waves (GWs). We use fully general relativistic hydrodynamics simulations to investigate how EM signatures correlate with BH spins, mass ratios, and the gaseous environment in this final phase of binary evolution. In all scenarios, we find some form of characteristic EM variability whose detailed pattern depends on the spins and binary mass ratios. Binaries in hot accretion flows exhibit a flare followed by a sudden drop in luminosity associated with the plunge and merger, as well as quasi-periodic oscillations correlated with the GWs during the inspiral. Conversely, circumbinary disk systems are characterized by a low luminosity of variable emission, suggesting challenging prospects for their detection.