Assisted Inspirals of Stellar Mass Black Holes Embedded in AGN Disks: Solving the "Final AU Problem"

TL;DR

This paper investigates how stellar mass black hole binaries in AGN disks can merge efficiently through disk-induced hardening mechanisms, potentially explaining observed gravitational wave events and predicting electromagnetic counterparts.

Contribution

It presents a detailed analysis of the formation and merger rates of black hole binaries in AGN disks, highlighting a viable channel for gravitational wave sources with electromagnetic signatures.

Findings

Estimated merger rate $\\sim 3~{ m yr}^{-1}~{ m Gpc}^{-3}$ consistent with LIGO observations.

Disk-induced hardening mechanisms effectively reduce binary separation within AGN lifetimes.

Potential electromagnetic counterparts due to super-Eddington accretion post-merger.

Abstract

We explore the evolution of stellar mass black hole binaries (BHBs) which are formed in the self-gravitating disks of active galactic nuclei (AGN). Hardening due to three-body scattering and gaseous drag are effective mechanisms that reduce the semi-major axis of a BHB to radii where gravitational waves take over, on timescales shorter than the typical lifetime of the AGN disk. Taking observationally-motivated assumptions for the rate of star formation in AGN disks, we find a rate of disk-induced BHB mergers ($\mathcal{R} \sim 3~{\rm yr}^{-1}~{\rm Gpc}^{-3}$, but with large uncertainties) that is comparable with existing estimates of the field rate of BHB mergers, and the approximate BHB merger rate implied by the recent Advanced LIGO detection of GW150914. BHBs formed thorough this channel will frequently be associated with luminous AGN, which are relatively rare within the sky error regions of future gravitational wave detector arrays. This channel could also possess a (potentially transient) electromagnetic counterpart due to super-Eddington accretion onto the stellar mass black hole following the merger.