Rapid and Bright Stellar-mass Binary Black Hole Mergers in Active Galactic Nuclei

TL;DR

This paper explores how stellar-mass binary black holes in active galactic nuclei can rapidly merge due to interactions with the accretion disk, leading to potentially detectable gravitational wave events and electromagnetic signals.

Contribution

It demonstrates that black hole binaries in active galactic nuclei can quickly merge within the disk, a process that enhances gravitational wave detection prospects and introduces new electromagnetic signatures.

Findings

Binary black holes can migrate and merge rapidly in AGN disks.

Estimated detection rate of ~20 such mergers per year by Advanced LIGO.

Binaries may accrete gas at hyper-Eddington rates, producing electromagnetic signals.

Abstract

The Laser Interferometer Gravitational-Wave Observatory, LIGO, found direct evidence for double black hole binaries emitting gravitational waves. Galactic nuclei are expected to harbor the densest population of stellar-mass black holes. A significant fraction ($\sim30\%$) of these black holes can reside in binaries. We examine the fate of the black hole binaries in active galactic nuclei, which get trapped in the inner region of the accretion disk around the central supermassive black hole. We show that binary black holes can migrate into and then rapidly merge within the disk well within a Salpeter time. The binaries may also accrete a significant amount of gas from the disk, well above the Eddington rate. This could lead to detectable X-ray or gamma-ray emission, but would require hyper-Eddington accretion with a few percent radiative efficiency, comparable to thin disks. We discuss implications for gravitational wave observations and black hole population studies. We estimate that Advanced LIGO may detect $\sim20$ such, gas-induced binary mergers per year.