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

Edward J Parkinson, Christian Knigge, James H Matthews, Knox S Long, Nick Higginbottom, Stuart A Sim, Samuel W Mangham

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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 RISCO lowers the wind ionization state sufficiently to produce UV absorption features for sightlines looking into the wind.
Original languageEnglish
Pages (from-to)5426–5443
JournalMonthly Notices of the Royal Astronomical Society
Volume510
Issue number4
Early online date10 Jan 2022
DOIs
Publication statusPublished - Mar 2022

Keywords

  • Space and Planetary Science
  • Astronomy and Astrophysics

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