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−1 recoil of a 10M 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 ∼ 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 ×10 erg/s, corresponding to ≈ 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.