Simulation of Binary-Single Interactions in AGN Disk I: Gas-Enhanced Binary Orbital Hardening

TL;DR

This study uses hydrodynamical and N-body simulations to show that gas in AGN disks significantly influences binary-single black hole interactions, leading to more compact binaries and potentially higher merger rates detectable by gravitational wave observatories.

Contribution

First comprehensive simulation of gas effects on binary-single black hole interactions in AGN disks, revealing increased compactness and merger rates.

Findings

Gas significantly alters the end states of binary-single interactions.

Higher gas density leads to more close encounters and more compact binaries.

Gas presence shortens GW merger timescales and increases merger rates.

Abstract

Stellar-mass binary black hole\,(BBH) mergers within the accretion disks of active galactic nuclei may contribute to gravitational wave\,(GW) events detected by grounded-based GW detectors. In particular, the interaction between a BBH and a single stellar-mass black hole\,(sBH), known as the binary-single interaction\,(BSI) process, can potentially lead to GW events with detectable non-zero eccentricity. Previous studies of the BSI process, which neglected the effects of gas, showed that BSIs contribute non-negligibly to GW events in a coplanar disk environment. In this work, we conduct a series of 2-dimensional hydrodynamical and N-body simulations to explore the BSI in a gas environment by coupling REBOUND with Athena++. We perform 360 simulation runs, spanning parameters in disk surface density \(\Sigma_0\) and impact parameter \(b\). We find that the gas-induced energy dissipation within the three-body system becomes significant if the encounter velocity between the sBHs is sufficiently large\,($\gg c_s$). Our simulation results indicate that approximately half of the end states of the BSI are changed by gas. Furthermore, at higher gas density, the number of close encounters during the BSI process will increase and the end-state BBHs tend to be more compact. Consequently, the presence of gas may shorten the GW merger timescale for end-state BBHs and increase the three-body merger rate.