First light from tidal disruption events

TL;DR

This study uses radiation-hydrodynamics simulations to explore the early emission and disc formation in tidal disruption events, revealing how initial shocks lead to observable luminosity and outflows.

Contribution

First radiation-hydrodynamics simulation of TDEs demonstrating how self-crossing shocks produce early emission and influence debris dynamics.

Findings

Injected matter circularizes promptly via secondary shocks.

Radiation diffuses efficiently, producing a luminous, thin accretion disc.

Outflows gain energy from radiation pressure, some becoming unbound.

Abstract

When a star comes too close to a supermassive black hole, it gets torn apart by strong tidal forces in a tidal disruption event, or TDE. Half of the elongated stream of debris comes back to the stellar pericenter where relativistic apsidal precession induces a self-crossing shock. As a result, the gas gets launched into an outflow that can experience additional interactions, leading to the formation of an accretion disc. We carry out the first radiation-hydrodynamics simulations of this process, making use of the same injection procedure to treat the self-crossing shock as in our previous adiabatic study (Bonnerot & Lu 2020). Two sets of realistic parameters of the problem are considered that correspond to different strengths of this initial interaction. In both cases, we find that the injected matter has its trajectories promptly circularized by secondary shocks taking place near the black hole. However, the generated internal energy efficiently diffuses away in the form of radiation, which results in a thin vertical profile of the formed disc. The diffusing photons promptly irradiate the surrounding debris until they emerge with a bolometric luminosity of $L\approx 10^{44} \, \rm erg\, s^{-1}$. Towards the self-crossing shock, diffusion is however slowed that results in a shallower luminosity increase, with a potentially significant component in the optical band. Matter launched to large distances continuously gains energy through radiation pressure, which can cause a significant fraction to become unbound. This work provides direct insight into the origin of the early emission from TDEs, which is accessed by a rapidly increasing number of observations.