Broken discs: warp propagation in accretion discs

TL;DR

This study simulates the viscous evolution of misaligned accretion discs around spinning black holes, revealing conditions under which nonlinear fluid effects cause the disc to break into two planes, affecting jet and system alignment.

Contribution

It introduces a simulation approach that incorporates effective viscosities consistent with fluid dynamics, showing how nonlinear effects can lead to disc breaking.

Findings

Disc breaking occurs for alpha <~ 0.3 and misalignment >~ 45 degrees.

Broken discs can persist and propagate outward before aligning.

Inner disc and jets may be misaligned with the orbital plane.

Abstract

We simulate the viscous evolution of an accretion disc around a spinning black hole. In general any such disc is misaligned, and warped by the Lense-Thirring effect. Unlike previous studies we use effective viscosities constrained to be consistent with the internal fluid dynamics of the disc. We find that nonlinear fluid effects, which reduce the effective viscosities in warped regions, can promote the breaking of the disc into two distinct planes. This occurs when the Shakura & Sunyaev dimensionless viscosity parameter alpha is <~ 0.3 and the initial angle of misalignment between the disc and hole is >~ 45 degrees. The break can be a long-lived feature, propagating outwards in the disc on the usual alignment timescale, after which the disc is fully co- or counter-aligned with the hole. Such a break in the disc may be significant in systems where we know the inclination of the outer accretion disc to the line of sight, such as some X-ray binaries: the inner disc, and so any jets, may be noticeably misaligned with respect to the orbital plane.