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

Optical line spectra of tidal disruption events from reprocessing in optically thick outflows
Optical line spectra of tidal disruption events from reprocessing in optically thick outflows
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 sight lines looking into the wind.
Accretion, accretion discs, Black hole physics, Transients: Tidal disruption events
0035-8711
5426-5443
Parkinson, Edward J.
2e087f5f-41c4-4cbb-9e9c-83c64f9add76
Knigge, Christian
ac320eec-631a-426e-b2db-717c8bf7857e
Matthews, James H.
8aa37525-32b9-460c-bb83-01c89269ac31
Long, Knox S.
2195d0ac-518d-4738-8e89-3e8e7a035a6c
Higginbottom, Nick
602bc39e-24c2-47fe-b39d-450681ec47af
Sim, Stuart A.
95ef9134-b7d7-4fbb-af7a-b00f8d5c8b65
Mangham, Samuel W.
c2053240-de45-4451-8cad-213930722d2e
Parkinson, Edward J.
2e087f5f-41c4-4cbb-9e9c-83c64f9add76
Knigge, Christian
ac320eec-631a-426e-b2db-717c8bf7857e
Matthews, James H.
8aa37525-32b9-460c-bb83-01c89269ac31
Long, Knox S.
2195d0ac-518d-4738-8e89-3e8e7a035a6c
Higginbottom, Nick
602bc39e-24c2-47fe-b39d-450681ec47af
Sim, Stuart A.
95ef9134-b7d7-4fbb-af7a-b00f8d5c8b65
Mangham, Samuel W.
c2053240-de45-4451-8cad-213930722d2e

Parkinson, Edward J., Knigge, Christian, Matthews, James H., Long, Knox S., Higginbottom, Nick, Sim, Stuart A. and Mangham, Samuel W. (2022) Optical line spectra of tidal disruption events from reprocessing in optically thick outflows. Monthly Notices of the Royal Astronomical Society, 510 (4), 5426-5443. (doi:10.1093/mnras/stac027).

Record type: Article

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 sight lines looking into the wind.

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In preparation date: 7 January 2021
Accepted/In Press date: 4 January 2022
Published date: 10 January 2022
Additional Information: Publisher Copyright: © 2022 The Author(s). arXiv:2201.01535
Keywords: Accretion, accretion discs, Black hole physics, Transients: Tidal disruption events

Identifiers

Local EPrints ID: 454346
URI: http://eprints.soton.ac.uk/id/eprint/454346
ISSN: 0035-8711
PURE UUID: 7edb1e2f-8247-44a6-b034-7f938bc6b716
ORCID for Edward J. Parkinson: ORCID iD orcid.org/0000-0003-3902-052X
ORCID for Samuel W. Mangham: ORCID iD orcid.org/0000-0001-7511-5652

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Date deposited: 07 Feb 2022 17:52
Last modified: 28 Apr 2022 02:20

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Contributors

Author: Edward J. Parkinson ORCID iD
Author: James H. Matthews
Author: Knox S. Long
Author: Nick Higginbottom
Author: Stuart A. Sim
Author: Samuel W. Mangham ORCID iD

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