Optical Flares from the Tidal Disruption of Stars by Massive Black Holes

TL;DR

This paper predicts optical and near-IR signatures of stellar tidal disruption flares by massive black holes, emphasizing their detectability in upcoming surveys and their importance for understanding black hole demographics and accretion physics.

Contribution

It provides detailed models of tidal disruption flare light curves and spectra, highlighting observable signatures and detection prospects in optical surveys.

Findings

Optical flares from tidal disruptions can mimic supernovae in duration and brightness.

Broad emission lines are strongest for black holes with masses around 10^5-10^6 solar masses.

Upcoming surveys like Pan-STARRS and LSST can detect multiple tidal disruption events annually.

Abstract

A star that wanders too close to a massive black hole (BH) is shredded by the BH's tidal gravity. Stellar gas falls back to the BH, releasing a flare of energy. In anticipation of upcoming transient surveys, we predict the light curves and spectra of tidal flares as a function of time, highlighting the unique signatures of tidal flares in the optical and near-IR. Some of the gas initially bound to the BH is likely blown away when the fallback rate is super-Eddington at early times. This outflow produces an optical luminosity comparable to that of a supernova; such events have durations of ~10 days and may have been missed in supernova searches that exclude the nuclear regions of galaxies. When the fallback rate subsides below Eddington, the gas accretes onto the BH via a thin disk whose emission peaks in the UV to soft X-rays. Some of this emission is reprocessed by the unbound stellar debris, producing a spectrum of very broad emission lines (with no corresponding narrow forbidden lines). These lines are strongest for BHs with MBH ~ 10^5 - 10^6 Msun and thus optical surveys are particularly sensitive to the lowest mass BHs in galactic nuclei. Calibrating our models to ROSAT and GALEX observations, we predict detection rates for Pan-STARRS, PTF, and LSST and highlight observational challenges in the optical. Pan-STARRS should detect at least several events per year--many more if current theoretical models of super-Eddington outflows are correct. These surveys will significantly improve our knowledge of stellar dynamics in galactic nuclei, the physics of super-Eddington accretion, the demography of intermediate mass BHs, and the role of tidal disruption in the growth of massive BHs.