Circumbinary Disk Accretion into Spinning Black Hole Binaries

TL;DR

This study uses approximate relativistic spacetime models and long-term magnetohydrodynamic simulations to analyze how black hole spins influence accretion rates in circumbinary disks near merger.

Contribution

It introduces a novel approximation method for the spacetime metric of close black hole binaries and investigates the impact of black hole spin orientation on disk accretion.

Findings

Aligned spins reduce accretion rate by 14%.

Counter-aligned spins increase accretion rate by 45%.

Other disk properties remain largely unaffected.

Abstract

Supermassive black hole binaries are likely to accrete interstellar gas through a circumbinary disk. Shortly before merger, the inner portions of this circumbinary disk are subject to general relativistic effects. To study this regime, we approximate the spacetime metric of close orbiting black holes by superimposing two boosted Kerr-Schild terms. After demonstrating the quality of this approximation, we carry out very long-term general relativistic magnetohydrodynamic simulations of the circumbinary disk. We consider black holes with spin dimensionless parameters of magnitude 0.9, in one simulation parallel to the orbital angular momentum of the binary, but in another anti-parallel. These are contrasted with spinless simulations. We find that, for a fixed surface mass density in the inner circumbinary disk, aligned spins of this magnitude approximately reduce the mass accretion rate by 14% and counter-aligned spins increase it by 45%, leaving many other disk properties unchanged.