Circumbinary discs around merging stellar-mass black holes

TL;DR

This paper investigates the evolution of circumbinary discs around merging stellar-mass black holes, analyzing how different disc conditions affect the potential electromagnetic signals produced during black hole mergers.

Contribution

It compares disc evolution under no torque and strong binary torque conditions, highlighting the impact of dead zones on mass retention near the recoil radius.

Findings

Discs without binary torque are unlikely to survive until merger unless a dead zone exists.

Strong binary torque prevents accretion, leading to more mass near the recoil radius.

Dead zones increase the mass close to the recoil radius, enhancing potential electromagnetic signals.

Abstract

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.