Shocks Power Tidal Disruption Events

TL;DR

This study uses relativistic simulations to show that debris in tidal disruption events forms an eccentric accretion flow rather than a compact disk, explaining observed energies without significant outflows.

Contribution

It demonstrates that most debris remains in an eccentric flow, challenging the assumption of prompt disk formation in TDEs, and links shock energy dissipation to observed radiation.

Findings

Most bound mass forms an eccentric accretion flow.

Shock dissipation accounts for observed radiated energy.

Minimal unbinding of debris occurs from shocks.

Abstract

Accretion of debris seems to be the natural mechanism to power the radiation emitted during a tidal disruption event (TDE), in which a supermassive black hole tears apart a star. However, this requires the prompt formation of a compact accretion disk. Here, using a fully relativistic global simulation for the long-term evolution of debris in a TDE with realistic initial conditions, we show that at most a tiny fraction of the bound mass enters such a disk on the timescale of observed flares. To "circularize" most of the bound mass entails an increase in the binding energy of that mass by a factor $\sim 30$; we find at most an order unity change. Our simulation suggests it would take a time scale comparable to a few tens of the characteristic mass fallback time to dissipate enough energy for "circularization". Instead, the bound debris forms an extended eccentric accretion flow with eccentricity $\simeq 0.4-0.5$ by $\sim 2$ fallback times. Although the energy dissipated in shocks in this large-scale flow is much smaller than the "circularization" energy, it matches the observed radiated energy very well. Nonetheless, the impact of shocks is not strong enough to unbind initially bound debris into an outflow.