Optical line spectra of tidal disruption events from reprocessing in optically thick outflows

TL;DR

This study models optically thick outflows in tidal disruption events to explain their optical/UV spectra, showing how wind properties influence observed features and spectral energy distribution.

Contribution

It introduces detailed Monte Carlo radiative transfer simulations of wind reprocessing in TDEs, linking wind characteristics to spectral features and broadening understanding of optical emission origins.

Findings

Winds efficiently reprocess disc emission, producing broad Balmer and helium lines.

Spectral energy distribution is significantly cooler than the accretion disc.

Wind properties like density and velocity strongly influence spectral features and ionization states.

Abstract

A significant number of tidal disruption events (TDEs) radiate primarily at optical and ultraviolet (UV) wavelengths, with only weak soft X-ray components. One model for this optical excess proposes that thermal X-ray emission from a compact accretion disc is reprocessed to longer wavelengths by an optically thick envelope. Here, we explore this reprocessing scenario in the context of an optically thick accretion disc wind. Using state-of-the-art Monte Carlo radiative transfer and ionization software, we produce synthetic UV and optical spectra for wind and disc-hosting TDEs. Our models are inspired by observations, spanning a realistic range of accretion rates and wind kinematics. We find that such outflows can efficiently reprocess the disc emission and produce the broad Balmer and helium recombination features commonly seen in TDEs and exhibit asymmetric red wings. Moreover, the characteristic colour temperature of the reprocessed spectral energy distribution (SED) is much lower than that of the accretion disc. We show explicitly how changes in black hole mass, accretion rate and wind properties affect the observed broadband SED and line spectrum. In general, slower, denser winds tend to reprocess more radiation and produce stronger Balmer emission. Most of the outflows we consider are too highly ionized to produce UV absorption features, but this is sensitive to the input SED. For example, truncating the inner disc at just 4 $R_{ISCO}$ lowers the wind ionization state sufficiently to produce UV absorption features for sight lines looking into the wind