The Bardeen-Petterson effect in accreting supermassive black-hole binaries: disc breaking and critical obliquity

TL;DR

This study links the Bardeen-Petterson effect to disc breaking in supermassive black-hole binaries, showing how it affects spin alignment and introduces complex disc behaviors through hydrodynamical simulations.

Contribution

It demonstrates that disc breaking occurs at a critical obliquity, affecting black hole spin alignment, and reveals complex phenomena like multiple disc breaks and stabilizing spiral arms.

Findings

Disc breaking occurs at a critical obliquity where warp solutions fail.

Hydrodynamical simulations confirm semi-analytic predictions and show complex disc behaviors.

Disc breaking can prevent or hinder black hole spin alignment during inspiral.

Abstract

The inspiral of supermassive black-hole binaries in gas-rich environment is driven by the presence of an accretion disc and viscous interactions tend to align the spin of the black holes with the orbital angular momentum of the disc. Recent work introduced a new iterative approach to describe the alignment process and the resulting non-linear evolution of the surrounding warped accretion disc. Their model predicted that black-hole spins reach either full alignment or a critical obliquity where solutions to the warp equations cease to exist. In this paper, we show that this critical region corresponds to the disc breaking phenomenon, where the disc is disrupted into two or more discrete sections. We use 3D hydrodynamical simulations to (i) recover the predictions of the semi-analytic model and (ii) unveil a richer phenomenology where the disc exhibits either unsuccessful, single and multiple breaks. We additionally identify hydrodynamic effects such as spiral arms that are able to stabilise the disc against breaking beyond criticality. Our results show that when disc breaking occurs, the ability of black holes and disc to align is compromised and in some cases even prevented as the binary inspirals.