  Modeling the late inspiral and merger of supermassive black holes is central
to understanding accretion processes and the conditions under which
electromagnetic emission accompanies gravitational waves. We use fully general
relativistic, hydrodynamics simulations to investigate how electromagnetic
signatures correlate with black hole spins, mass ratios, and the gaseous
environment in this final phase of binary evolution. In all scenarios, we find
some form of characteristic electromagnetic variability whose 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
gravitational waves during the inspiral. Conversely, circumbinary disk systems
are characterized by a low luminosity of variable emission, suggesting
challenging prospects for their detection.
