Accretion into the Central Cavity of a Circumbinary Disk

TL;DR

This study uses 2D hydrodynamical simulations to analyze how binary black holes influence accretion streams and rates in circumbinary disks, revealing how mass ratio affects accretion suppression and variability patterns.

Contribution

It provides new insights into the dependence of accretion dynamics on binary mass ratio, including suppression factors and periodic modulation, using extensive simulations across different q values.

Findings

Accretion into the cavity is suppressed by up to a factor of 5.

Maximum suppression occurs at q≈0.05.

Accretion rate variability depends on the mass ratio q.

Abstract

A near-equal mass binary black hole can clear a central cavity in a circumbinary accretion disk; however, previous works have revealed accretion streams entering this cavity. Here we use 2D hydrodynamical simulations to study the accretion streams and their periodic behavior. In particular, we perform a suite of simulations, covering different binary mass ratios $q=M_2/M_1$ in the range $0.01 \leq q \leq 1$. In each case, we follow the system for several thousand binary orbits, until it relaxes to a stable accretion pattern. We find the following results: (i) while the binary is efficient in maintaining a low-density cavity, the time-averaged mass accretion rate into the cavity, through narrow coherent accretion streams, is suppressed by at most a factor of $\sim 5$, compared to a disk with a single BH with the same mass; (ii) the largest suppression occurs for $q\approx 0.05$; binaries whose mass ratios are either lower or higher both suppress accretion less significantly; (iii) for $q \gtrsim 0.05$, the accretion rate is strongly modulated by the binary, and depending on the precise value of $q$, the power spectrum of the accretion rate shows either one, two, or three distinct periods; and (v) for $q \lesssim 0.05$, the accretion rate becomes steady, with no time-variations. Most binaries produced in galactic mergers are expected to have $q\gtrsim 0.05$. If the luminosity of these binaries tracks their accretion rate, then a periodogram of their light-curve could help in their identification, and to constrain their mass ratio and disk properties.