Filling the disk hollow following binary black hole merger: The transient accretion afterglow

Understand

Tidal torques from a binary black hole (BHBH) empty out the central regions in any circumbinary gaseous accretion disk.

• The balance between tidal torques and viscosity maintain the inner edge of the disk at a radius r ~ 1.5a -- 2a, where a is the binary semimajor axis.
• Eventually, the inspiraling binary decouples from disk and merges, leaving behind a central hollow ("donut hole") in the disk orbiting the remnant black hole.
• We present a simple, time-dependent, Newtonian calculation that follows the secular (viscous) evolution of the disk as it fills up the hollow down to the black hole innermost stable circular orbit and then relaxes to stationary equilibrium.