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

## TL;DR

This paper reports the discovery of high-velocity broad absorption lines in a TDE, indicating that fast outflows are common in such events and can be observed through UV and optical spectra.

## Contribution

It presents the first detection of highly blueshifted broad Balmer and metastable helium absorption lines in a TDE, linking outflows to TDE phenomena and expanding understanding of their spectral features.

## Key findings

- High-velocity absorption lines are observed in TDE AT2018zr.
- Outflows are likely responsible for transient absorption features.
- High association rate of BALs suggests outflows are common in TDEs.

## 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$4640, HeII$\lambda4686$, H$\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.