Gravitational Wave Decoupling in Retrograde Circumbinary Disks

TL;DR

This paper investigates how supermassive black hole binaries interact with retrograde circumbinary disks during gravitational wave-driven inspiral, revealing unique dynamics, electromagnetic signatures, and instabilities distinct from prograde disks.

Contribution

It provides the first detailed numerical analysis of binary-disk decoupling in retrograde disks, highlighting differences in cavity size, EM emissions, and instabilities compared to prograde disks.

Findings

Decoupling points are similar in prograde and retrograde disks.

Retrograde disks have smaller central cavities, leading to higher-frequency EM emissions.

Retrograde disks exhibit quasi-periodic flaring due to unique instabilities.

Abstract

We present a study of the late-time interaction between supermassive black hole binaries and retrograde circumbinary disks during the period of gravitational wave-driven inspiral. While mergers in prograde disks have received extensive study, retrograde disks offer distinct dynamics that could promote mergers and produce unique observational signatures. Through numerical simulations, we explore the process of binary-disk decoupling, where the binary's orbital decay rate is faster than the disk's viscous response rate. We find the point of decoupling to be comparable in prograde and retrograde disks, suggesting that any associated electromagnetic (EM) signatures will be produced at comparable times preceding merger. However, we find smaller central cavities for retrograde disks, likely leading to higher-frequency EM emissions and shorter post-merger rebrightening timescales compared to their prograde counterparts. Additionally, we identify quasi-periodic flaring due to instabilities unique to low-viscosity retrograde disks, which may produce distinctive EM signatures.