Binary black hole accretion across the disk and gravitational wave-driven regimes

Supermassive black hole binaries typically form at separations far too wide for the emission of gravitational waves to drive them to coalescence. Both dynamical interactions with stars and other compact objects and interaction with a circumbinary accretion disk may expedite black hole mergers. We will present state-of-the-art simulations of circumbinary accretion, bridging the gap between the disk- and gravitational wave-driven regimes. Our results concern the orbital evolution and observable characteristics of supermassive black hole binaries of varying mass ratios as they evolve towards detectability by LISA. These include systems with mass ratios spanning the range from 1:1 to 100:1; and those with disk aspect ratios between 0.1 and 0.02, approaching those expected for realistic systems. Additionally, we will demonstrate the morphological changes of accretion flows around SMBH binaries as they decouple from their circumbinary disks in the gravitational wave-dominated regime and evolve into the LISA band.