Spectroscopic Signatures of the Tidal Disruption of Stars by Massive Black Holes

TL;DR

This paper predicts the spectral signatures of tidal disruption events caused by massive black holes, highlighting their UV absorption lines and discussing how to distinguish them from supernovae in optical surveys.

Contribution

It provides detailed predictions of the spectra and evolution of tidal disruption events, aiding their identification in optical transient surveys.

Findings

Spectral lines are strongly blueshifted and broad, prominent in UV wavelengths.

Tidal disruption events are less common than nuclear supernovae but can be distinguished with high-resolution imaging.

Predicted spectral features last about one month for a 10^6 solar mass black hole.

Abstract

During the tidal disruption of a star by a massive black hole (BH) of mass MBH <~ 10^7 Msun, stellar debris falls back to the BH at a rate well above the Eddington rate. A fraction of this gas is subsequently blown away from the BH, producing an optically bright flare of radiation. We predict the spectra and spectral evolution of tidal disruption events, focusing on the photoionized gas outside this outflow's photosphere. The spectrum will show absorption lines that are strongly blueshifted relative to the host galaxy, very broad (0.01-0.1c), and strongest at UV wavelengths (e.g., C IV, Ly alpha, O VI), lasting ~ 1 month for a 10^6 Msun BH. Meanwhile, supernovae in galactic nuclei are a significant source of confusion in optical surveys for tidal disruption events: we estimate that nuclear Type Ia supernovae are two orders of magnitude more common than tidal disruption events at z ~ 0.1 for ground-based surveys. Nuclear Type II supernovae occur at a comparable rate but can be excluded by pre-selecting red galaxies. Supernova contamination can be reduced to a manageable level using high-resolution follow-up imaging with adaptive optics or the Hubble Space Telescope. Our predictions should help optical transient surveys capitalize on their potential for discovering tidal disruption events.