  A circumbinary disc around a pair of merging stellar-mass black holes may be
shocked and heated during the recoil of the merged hole, causing a
near-simultaneous electromagnetic counterpart to the gravitational wave event.
The shocks occur around the recoil radius, where the disc orbital velocity is
equal to the recoil velocity. The amount of mass present near this radius at
the time of the merger is critical in determining how much radiation is
released. We explore the evolution of a circumbinary disc in two limits. First,
we consider an accretion disc that feels no torque from the binary. The disc
does not survive until the merger unless there is a dead zone, a region of low
turbulence. Even with the dead zone, the surface density in this case may be
small. Second, we consider a disc that feels a strong binary torque that
prevents accretion on to the binary. In this case there is significantly more
mass in regions of interest at the time of the merger. A dead zone in this disc
increases the mass close to the recoil radius. For typical binary-disc
parameters we expect accretion to be significantly slowed by the resonant
torque from the binary, and for a dead zone to be present. We conclude that
provided significant mass orbits the binary after the formation of the black
hole binary and that the radiation produced in recoil shocks can escape the
flow efficiently, there is likely to be an observable electromagnetic signal
from black hole binary mergers.
