The evolution of a supermassive retrograde binary embedded in an accretion disk

TL;DR

This paper investigates the dynamics and accretion processes of a supermassive binary black hole system embedded in an accretion disk, highlighting differences from prograde cases and implications for gravitational wave detection.

Contribution

It presents a detailed analysis of gap formation, migration timescales, and accretion rates in retrograde supermassive binaries, extending understanding beyond prograde scenarios.

Findings

Gap formation mechanism differs from prograde case.

Inward migration timescale is proportional to M_p/Ṁ.

Accretion onto the perturber is suppressed for q<<1, but increases with gravitational wave effects.

Abstract

In this note we discuss the main results of a study of a massive binary with unequal mass ratio, q, embedded in an accretion disk, with its orbital rotation being opposed to that of the disk. When the mass ratio is sufficiently large, a gap opens in the disk, but the mechanism of gap formation is very different from the prograde case. Inward migration occurs on a timescale of t_ev ~ M_p/(dot M), where M_p is the mass of the less massive component (the perturber), and dot M is the accretion rate. When q<< 1, the accretion takes place mostly onto the more massive component, with the accretion rate onto the perturber being smaller than, or of order of, q^(1/3)M. However, this rate increases when supermassive binary black holes are considered and gravitational wave emission is important. We estimate a typical duration of time for which the accretion onto the perturber and gravitational waves could be detected.