Discovery of Highly Blueshifted Broad Balmer and Metastable Helium Absorption Lines in a Tidal Disruption Event

Abstract

We report the discovery of non-stellar hydrogen Balmer and metastable helium absorption lines accompanying a transient, high-velocity (0.05$c$) broad absorption line (BAL) system in the optical spectra of the tidal disruption event (TDE) AT2018zr ($z=0.071$). In the HST UV spectra, absorption of high- and low-ionization lines are also present at this velocity, making AT2018zr resemble a low-ionization broad absorption line (LoBAL) QSO. We conclude that these transient absorption features are more likely to arise in fast outflows produced by the TDE than absorbed by the unbound debris. In accordance with the outflow picture, we are able to reproduce the flat-topped H$\alpha$ emission in a spherically expanding medium, without invoking the typical prescription of an elliptical disk. We also report the appearance of narrow ($\sim$1000~km~s${}^{-1}$) NIII$\lambda$4640, HeII$\lambda 4686$, H$\alpha$, and H$\beta$, emission in the late-time optical spectra of AT2018zr, which may be a result of UV continuum hardening at late time as observed by Swift. Including AT2018zr, we find a high association rate (3 out of 4) of BALs in the UV spectra of TDEs. This suggests that outflows may be ubiquitous among TDEs and may be less sensitive to viewing angle effects compared to QSO outflows.

20 pages, 11 figures, submitted to ApJ

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• Measuring black hole masses from tidal disruption events and testing the $M_{\mathrm{BH}}-{\sigma }_{\ast }$ relation
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• AT2018dyk Revisited: a Tidal Disruption Event Candidate with Prominent Infrared Echo and Delayed X-ray Emission in a LINER Galaxy
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• Modeling Multiple X-Ray Reflection in Super-Eddington Winds
• A multi-dimensional view of a unified model for TDEs
• The Effect of Impact Parameter on Tidal Disruption Events
• JWST Discovery of a High-Redshift Tidal Disruption Event Candidate in COSMOS-Web
• eROSITA-RU Tidal Disruption Events with Keck-I/LRIS: Sample Selection, Optical Properties, and Host Galaxy Demographics
• Partial, zombie, and full tidal disruption of stars by supermassive black holes
• A detailed spectroscopic study of Tidal Disruption Events
• Numerical Studies on the Radio Afterglows in TDE: Forward Shock
• Ionization and dissociation induced fragmentation of a tidally disrupted star into planets around a supermassive black hole
• Super-Accreting Active Galactic Nuclei as Neutrino Sources
• Dynamical Unification of Tidal Disruption Events
• Fast giant flares in discs around supermassive black holes