Hydrodynamical Response of a Circumbinary Gas Disk to Black Hole Recoil
  and Mass Loss
  Finding electromagnetic (EM) counterparts of future gravitational wave (GW)
sources would bring rich scientific benefits. A promising possibility, in the
case of the coalescence of a super-massive black hole binary (SMBHB), is that
prompt emission from merger-induced disturbances in a supersonic circumbinary
disk may be detectable. We follow the post-merger evolution of a thin,
zero-viscosity circumbinary gas disk with two-dimensional simulations, using
the hydrodynamic code FLASH. We analyze perturbations arising from the 530 km/s
recoil of a 10^6 M_sun binary, oriented in the plane of the disk, assuming
either an adiabatic or a pseudo-isothermal equation of state for the gas. We
find that a single-armed spiral shock wave forms and propagates outward,
sweeping up about 20% of the mass of the disk. The morphology and evolution of
the perturbations agrees well with those of caustics predicted to occur in a
collisionless disk. Assuming that the disk radiates nearly instantaneously to
maintain a constant temperature, we estimate the amount of dissipation and
corresponding post-merger light-curve. The luminosity rises steadily on the
time-scale of months, and reaches few times 10^{43} erg/s, corresponding to
about 10% of the Eddington luminosity of the central SMBHB. We also analyze the
case in which gravitational wave emission results in a 5% mass loss in the
merger remnant. The mass-loss reduces the shock overdensities and the overall
luminosity of the disk by 15-20%, without any other major effects on the spiral
shock pattern.
