Pre-peak Emission in Tidal Disruption Events

TL;DR

This study uses radiation hydrodynamic simulations to explore the early emission mechanisms in tidal disruption events, revealing how stream collisions influence outflows, luminosity, and gas circularization.

Contribution

It provides new insights into pre-peak emission processes, especially the role of stream-stream collisions and outflows in delaying gas circularization during tidal disruption events.

Findings

Stream-stream collisions can occur multiple times at super-Eddington fallback rates.

Outflows driven by collisions can reach up to 9 times the Eddington accretion rate.

Luminosity remains sub-Eddington and is weakly correlated with accretion rate early on.

Abstract

The rising part of a tidal disruption event light curve provides unique insight into early emission and the onset of accretion. Various mechanisms are proposed to explain the pre-peak emission, including shocks from debris interaction and reprocessing of disk emission. We study the pre-peak emission and its influence on the gas circularization by a series of gray radiation hydrodynamic simulations with varying black hole mass. We find that given a super-Eddington fallback rate of 10\dot{M}_{Edd}, the stream-stream collision can occur multiple times and drive strong outflows of up to 9\dot{M}_{Edd}. By dispersing gas to \gtrsim 100rs, the outflow can delay gas circularization and leads to sub-Eddington accretion rates during the first few stream-stream collisions. The stream-stream collision shock and circularization shock can sustain a luminosity of ~10^{44}erg/s for days. The luminosity is generally sub-Eddington and shows a weak correlation with accretion rate at early time. The outflow is optically thick, yielding a reprocessing layer with a size of ~10^{14} cm and photospheric temperature of ~4\times10^{4}K.