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

TL;DR

This paper investigates the spectral features of tidal disruption event candidates, finds inconsistencies with traditional TDE models, and proposes a light-echo photoionization model along with alternative explanations like supernovae or faint AGNs.

Contribution

It introduces a novel light-echo photoionization model to explain spectral features and explores alternative origins for nuclear flares beyond TDEs.

Findings

Observed line velocities are lower than expected for TDE debris.

AGN-like illumination cannot reproduce observed equivalent widths.

A light-echo model explains spectral features better than standard TDE models.

Abstract

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. We use exhaustive photoionization modelling to map the domain of fluxes and cloud properties that yield (e.g.) a He-overbright state where a large HeII(4686A)/H$\alpha$ line-ratio creates an illusion of helium enrichment. Although observed line ratios occur in a plausible minority of cases, AGN-like illumination can not reproduce the observed equivalent widths. We therefore propose to explain these properties by a light-echo photoionization model: the initial flash of a hot blackbody (detonation) excites BLR clouds, which are then seen superimposed on continuum from a later, expanded, cooled stage of the central luminous source. The implied cloud mass is substellar, which may be inconsistent with a TDE. Given these and other inconsistencies with TDE models (e.g. host-galaxies distribution) we suggest to also consider alternative origins for these nuclear flares, which we briefly discuss (e.g. nuclear supernovae and starved/subluminous AGNs).