Magnetohydrodynamical simulations of a tidal disruption in general relativity

TL;DR

This study uses general relativistic magnetohydrodynamical simulations to analyze the complex processes of tidal disruption of a red dwarf by a massive black hole, revealing shock formation, debris circularization, and low radiative efficiency.

Contribution

First detailed GRMHD simulations of a tidal disruption event with a red dwarf and a supermassive black hole, highlighting shock dynamics and debris evolution.

Findings

Relativistic precession causes self-crossing shocks.

Debris forms a turbulent, marginally bound disc.

Low radiative efficiency due to photon trapping.

Abstract

We perform hydro- and magnetohydrodynamical general relativistic simulations of a tidal disruption of a $0.1\,M_\odot$ red dwarf approaching a $10^5\,M_\odot$ non-rotating massive black hole on a close (impact parameter $\beta=10$) elliptical (eccentricity $e=0.97$) orbit. We track the debris self-interaction, circularization, and the accompanying accretion through the black hole horizon. We find that the relativistic precession leads to the formation of a self-crossing shock. The dissipated kinetic energy heats up the incoming debris and efficiently generates a quasi-spherical outflow. The self-interaction is modulated because of the feedback exerted by the flow on itself. The debris quickly forms a thick, almost marginally bound disc that remains turbulent for many orbital periods. Initially, the accretion through the black hole horizon results from the self-interaction, while in the later stages it is dominated by the debris originally ejected in the shocked region, as it gradually falls back towards the hole. The effective viscosity in the debris disc stems from the original hydrodynamical turbulence, which dominates over the magnetic component. The radiative efficiency is very low because of low energetics of the gas crossing the horizon and large optical depth that results in photon trapping.