Introduction
Supermassive black hole binaries can form in magnetized plasma following galaxy mergers, via bar-mode instability in rapidly rotating supermassive stars, or by other dynamical processes. After formation, a combination of dynamical friction and gas-driven migration is likely to catalyze the binary inspiral into the gravitational radiation-driven regime. The exciting prospect of a simultaneous observation of both electromagnetic (EM) and gravitational waves (GWs) arising from accreting binary BHBHs makes these systems prime targets in the era of multi-messenger astronomy. We study the effects of the binary mass ratio on the disk near decoupling. We vary the BHBH binary mass ratio q = M1/M2 < 1, considering 1:1, 1:2, 1:4, 1:8 and 1:10 mass ratios. On this website, we visualize mass ratios of 1:1, 1:4, and 1:10.
The simulations are performed either with or without radiative cooling. Without cooling, the binary tidal and the viscous torques act gradually to heat and puff up the disk. "Advective" cooling is accounted for in all the simulations. However radiative cooling is necessary to reach a true steady state. Realistic cooling depends on complicated microphysics not modeled here, but they will be treated in future studies. Instead, we implement a simple cooling leakage scheme. This scheme is, strictly speaking, only valid in the optically thin regime. While this is a very crude approach to radiative cooling, treating both extremely rapid radiative cooling as well as no radiative cooling can help bracket the possibilities.
