Reaction of Accretion Disks to Abrupt Mass Loss During Binary Black Hole Merger

TL;DR

This paper investigates how accretion disks around merging black holes respond to sudden mass loss, finding that observable luminosity decreases are detectable, especially with future X-ray or radio telescopes, despite shocks being limited to thin disks.

Contribution

It provides an analytical and numerical evaluation of accretion disk reactions to abrupt black hole mass loss, highlighting the potential for observable luminosity decreases.

Findings

Shocks occur only in geometrically thin disks with significant mass loss.

Luminosity variations are likely obscured by natural disk fluctuations.

Decreases in luminosity due to inner disk retreat are potentially detectable.

Abstract

The association of an electromagnetic signal with the merger of a pair of supermassive black holes would have many important implications. For example, it would provide new information about gas and magnetic field interactions in dynamical spacetimes as well as a combination of redshift and luminosity distance that would enable precise cosmological tests. A proposal first made by Bode & Phinney (2007) is that because radiation of gravitational waves during the final inspiral and merger of the holes is abrupt and decreases the mass of the central object by a few percent, there will be waves in the disk that can steepen into shocks and thus increase the disk luminosity in a characteristic way. We evaluate this process analytically and numerically. We find that shocks only occur when the fractional mass loss exceeds the half-thickness (h/r) of the disk, hence significant energy release only occurs for geometrically thin disks which are thus at low Eddington ratios. This strongly limits the effective energy release, and in fact our simulations show that the natural variations in disk luminosity are likely to obscure this effect entirely. However, we demonstrate that the reduction of luminosity caused by the retreat of the inner edge of the disk following mass loss is potentially detectable. This decrease occurs even if the disk is geometrically thick, and lasts for a duration on the order of the viscous time of the modified disk. Observationally, the best prospect for detection would be a sensitive future X-ray instrument with a field of view of on the order of a square degree, or possibly a wide-field radio array such as the Square Kilometer Array, if the disk changes produce or interrupt radio emission from a jet.