Suppression of the accretion rate in thin discs around binary black holes

TL;DR

This study uses 3D SPH simulations to show that thinner accretion discs around binary black holes significantly suppress mass inflow, impacting electromagnetic signals, gravitational waves, and black hole recoil in astrophysical systems.

Contribution

It demonstrates how disc thickness influences accretion rates in binary black hole systems, revealing a linear suppression in thinner discs, with implications for astrophysical observations.

Findings

Thick discs allow gas to penetrate the cavity easily.

Thinner discs suppress accretion proportionally to H/R.

Results are applicable to both supermassive and stellar-mass black hole binaries.

Abstract

We present three-dimensional Smoothed Particle Hydrodynamics (SPH) simulations investigating the dependence of the accretion rate on the disc thickness around an equal-mass, circular black hole binary system. We find that for thick/hot discs, with $H/R\gtrsim 0.1$, the binary torque does not prevent the gas from penetrating the cavity formed in the disc by the binary (in line with previous investigations). The situation drastically changes for thinner discs, in this case the mass accretion rate is suppressed, such that only a fraction (linearly dependent on $H/R$) of the available gas is able to flow within the cavity and accrete on to the binary. Extrapolating this result to the cold and thin accretion discs expected around supermassive black hole binary systems implies that this kind of systems accretes less material than predicted so far, with consequences not only for the electromagnetic and gravitational waves emissions during the late inspiral phase but also for the recoil speed of the black hole formed after binary coalescence, thus influencing also the evolutionary path both of the binary and of the host galaxy. Our results, being scale-free, are also applicable to equal mass, circular binaries of stellar mass black holes, such as the progenitor of the recently discovered gravitational wave source GW150914.