Hydrodynamical Response of a Circumbinary Gas Disk to Black Hole Recoil and Mass Loss

TL;DR

This study uses hydrodynamic simulations to explore how a circumbinary gas disk responds to black hole recoil and mass loss, predicting observable electromagnetic signals following black hole mergers.

Contribution

It provides the first detailed hydrodynamic modeling of disk perturbations caused by black hole recoil and mass loss, linking these to potential electromagnetic counterparts.

Findings

A spiral shock wave forms and propagates outward after recoil.

The luminosity from the disk increases to about 10% of the Eddington luminosity.

Mass loss reduces shock strength and luminosity by 15-20%.

Abstract

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.