Lense-Thirring precession around supermassive black holes during tidal disruption events

TL;DR

This paper models the Lense-Thirring precession of accretion discs around supermassive black holes during tidal disruption events, exploring precession periods, conditions for rigid precession, and alignment processes.

Contribution

It introduces a simple model for thick accretion discs in TDEs to analyze precession behavior and alignment mechanisms in the context of black hole spin and disc inclination.

Findings

Precession period depends on system parameters.

Conditions for rigid precession are identified.

Mechanisms for disc and black hole alignment are discussed.

Abstract

A tidal disruption event (TDE) occurs when a star wanders close enough to a black hole to be disrupted by its tidal force. The debris of a tidally disrupted star are expected to form an accretion disc around the supermassive black hole. The light curves of these events sometimes show a quasi-periodic modulation of the flux that can be associated with the precession of the accretion disc due to the Lense-Thirring ("frame-dragging") effect. Since the initial star orbit is in general inclined with respect to the black hole spin, this misalignment combined with the Lense-Thirring effect leads to a warp in the disc. In this paper we provide a simple model of the system composed by a thick and narrow accretion disc surrounding a spinning supermassive black hole, with the aim to: (a) compute the expected precession period as a function of the system parameters, (b) discuss the conditions that have to be satisfied in order to have rigid precession, (c) investigate the alignment process, highlighting how different mechanisms play a role leading the disc and the black hole angular momenta into alignment.