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

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.

Dynamical Unification of Tidal Disruption Events

“…Some theoretical work has been done to understand TDE spectra. In particular, Parkinson et al (2020Parkinson et al ( , 2022 have modeled TDE UV/optical continuum and line spectra also based on a reprocessing model. They adopted a biconical 2D disk wind toy model, somewhat similar to ours, but they focused on the phases after peak when the accretion rate and wind mass rate both drop below the Eddington level.…”

“…Therefore, we could have underestimated the optical emissions observed from low inclinations. In addition, as shown by Parkinson et al (2020Parkinson et al ( , 2022, implementing 2D or 3D radiative transfer calculations can allow one to follow photons along more realistic paths, which induce effects such as the reduction of photon trapping in the wind. We will also work toward including 2D/3D radiative transfer calculations in our postprocessing simulation.…”

“…Many theoretical models have been proposed to explain these TDE emission properties (Roth et al 2020). While the X-ray emissions have been predicted by classical accretion disk models (Ulmer 1999), UV/optical emissions are argued to be produced from either the reprocessing of X-ray photons in an extended envelope or outflows (Loeb & Ulmer 1997;Strubbe & Quataert 2009;Lodato & Rossi 2011;Coughlin & Begelman 2014;Guillochon et al 2014;Roth et al 2016;Parkinson et al 2020Parkinson et al , 2022 or the shocks powered by debris stream self-intersection (Piran et al 2015;Bonnerot et al 2021). The late-time rebrightening of X-rays in TDEs can be accounted for by either a change in the disk morphology as the accretion rate declines from super-Eddington to sub-Eddington or the delayed onset of accretion (van Velzen et al 2019;Hayasaki & Jonker 2021).…”

The Tidal Disruption Event AT2021ehb: Evidence of Relativistic Disk Reflection, and Rapid Evolution of the Disk–Corona System

“…In the reprocessing model, the nature of the reprocessing layer could originate from either a disk wind (Strubbe & Quataert 2009;Miller 2015;Dai et al 2018;Parkinson et al 2022;Thomsen et al 2022) or an outflow from the selfcrossing shock (Jiang et al 2016;Lu & Bonnerot 2020). The outflow scenario is favored for two reasons.…”

The origin of the optical/ultraviolet emission of optical/ultraviolet tidal disruption events

“…In this scenario, the shock/emission radii can be consistent with that inferred from observations. In the second scenario, the soft X-ray/EUV emission generated by the debirs accretion onto the black hole is reprocessed into optical/NUV bands by an extended elliptical disk (Liu et al 2017(Liu et al , 2021Wevers et al 2022) or an envelope (Loeb & Ulmer 1997;Coughlin & Begelman 2014;Roth et al 2016) or an outflow (Strubbe & Quataert 2009;Lodato & Rossi 2011;Metzger & Stone 2016;Parkinson et al 2022) surrounding the central debris disk. The 'reprocessing' outflows may have two origins.…”