Tearing up the disc: how black holes accrete

TL;DR

This paper demonstrates that Lense-Thirring precession causes tilted accretion discs around spinning black holes to break into separate planes, leading to rapid infall and potential observable phenomena like QPOs.

Contribution

It reveals how disc breaking occurs due to relativistic precession in realistic accretion scenarios, affecting black hole feeding and observable signals.

Findings

Disc breaking occurs for tilt angles between 45° and 135°.

Broken discs facilitate rapid gas infall and accretion.

Observable phenomena such as QPOs may result from disc breaking.

Abstract

We show that in realistic cases of accretion in active galactic nuclei or stellar-mass X-ray binaries, the Lense-Thirring effect breaks the central regions of tilted accretion discs around spinning black holes into a set of distinct planes with only tenuous flows connecting them. If the original misalignment of the outer disc to the spin axis of the hole is $45^{\circ} \lesssim \theta \lesssim 135^{\circ}$, as in $\sim 70$% of randomly oriented accretion events, the continued precession of these discs sets up partially counter-rotating gas flows. This drives rapid infall as angular momentum is cancelled and gas attempts to circularize at smaller radii. Disc breaking close to the black hole leads to direct dynamical accretion, while breaking further out can drive gas down to scales where it can accrete rapidly. For smaller tilt angles breaking can still occur, and may lead to other observable phenomena such as QPOs. For such effects not to appear, the black hole spin must in practice be negligibly small, or be almost precisely aligned with the disc. Qualitatively similar results hold for any accretion disc subject to a forced differential precession, such as an external disc around a misaligned black hole binary.