Variability in tidal disruption events: gravitationally unstable streams

TL;DR

This paper presents detailed simulations of a solar-mass star's tidal disruption by a supermassive black hole, revealing that stream self-gravity causes debris recollapse and variability in fallback rates, impacting observed tidal disruption events.

Contribution

First comprehensive simulation covering full evolution of a tidal disruption event, highlighting the role of stream self-gravity in debris recollapse and fallback rate variability.

Findings

Fallback rate follows t^{-5/3} power-law on average.

Self-gravity causes debris to recollapse into bound fragments.

Fallback rate exhibits significant variability around the average.

Abstract

We present simulations of the tidal disruption of a solar mass star by a $10^6M_{\odot}$ black hole. These, for the first time, cover the full time evolution of the tidal disruption event, starting well before the initial encounter and continuing until more than 90% of the bound material has returned to the vicinity of the hole. Our results are compared to the analytical prediction for the rate at which tidally-stripped gas falls back. We find that, for our chosen parameters, the overall scaling of the fallback rate, $\dot{M}_{\rm{fb}}$, closely follows the canonical $t^{-5/3}$ power-law. However, our simulations also show that the self-gravity of the tidal stream, which dominates the tidal gravity of the hole at large distances, causes some of the debris to recollapse into bound fragments before returning to the hole. This causes $\dot{M}_{\rm{fb}}$ to vary significantly around the $t^{-5/3}$ average. We discuss the implications of our findings in the context of the event Swift J1644+57.