Optical Transients from the Unbound Debris of Tidal Disruption

TL;DR

This paper proposes that unbound debris from tidal disruption events can produce observable optical transients powered by hydrogen recombination, providing a new signature to identify such events.

Contribution

It introduces a model for optical emission from unbound debris, highlighting the role of hydrogen recombination in producing detectable transients.

Findings

Optical transients occur ~1 week after disruption.

Transients last 3-5 days with luminosities of 10^40-10^42 erg/s.

Recombination-driven emission can be a signature of tidal disruption events.

Abstract

In the tidal disruption of a star by a black hole, roughly half of the stellar mass becomes bound and falls into the black hole, while the other half is ejected at high velocity. Several previous studies have considered the emission resulting from the accretion of bound material; we consider the possibility that the unbound debris may also radiate once it has expanded and become transparent. We show that the gradual energy input from hydrogen recombination compensates for adiabatic loses over significant expansion factors. The opacity also drops dramatically with recombination, and the internal energy can be radiated by means of a cooling-transparency wave propagating from the surface layers inward. The result is a brief optical transient occurring ~1 week after disruption and lasting 3-5 days with peak luminosities of 10^40-10^42 ergs/s, depending on the mass of the disrupted star. These recombination powered transients should accompany the x-ray/ultraviolet flare from the accretion of bound material, and so may be a useful signature for discriminating tidal disruption events, especially for lower and intermediate mass black holes.