Spectral features of tidal-disruption candidates and alternative origins for such transient flares

Understand

UV and optically selected candidates for stellar tidal disruption events (TDE) often exhibit broad spectral features (HeII emission, H$\alpha$ emission, or absorption lines) on a blackbody-like continuum (1e4K<T<1e5K).

• The lines presumably emit from TDE debris or circumnuclear clouds photoionized by the flare.
• Line velocities however are much lower than expected from a stellar disruption by supermassive black hole (SMBH), and are somewhat faster than expected for the broad line region (BLR) clouds of a persistently active galactic nucleus (AGN).
• The distinctive spectral states are not strongly related to observed luminosity and velocity, nor to SMBH mass estimates.