Discovery of a Fast Iron Low-ionization Outflow in the Early Evolution of the Nearby Tidal Disruption Event AT 2019qiz

Hung, Tiara and Foley, Ryan J. and Veilleux, S. and Cenko, S. B. and Dai, Jane L. and Auchettl, Katie and Brink, Thomas G. and Dimitriadis, Georgios and Filippenko, Alexei V. and Gezari, S. and Holoien, Thomas W.-S. and Kilpatrick, Charles D. and Mockler, Brenna and Piro, Anthony L. and Ramirez-Ruiz, Enrico and Rojas-Bravo, César and Siebert, Matthew R. and van Velzen, Sjoert and Zheng, WeiKang (2021) Discovery of a Fast Iron Low-ionization Outflow in the Early Evolution of the Nearby Tidal Disruption Event AT 2019qiz. The Astrophysical Journal, 917 (1): 9. ISSN 0004-637X

Abstract

We report the results of ultraviolet (UV) and optical photometric and spectroscopic analysis of the tidal disruption event (TDE) AT 2019qiz. Our follow-up observations started <10 days after the source began to brighten in the optical and lasted for a period of six months. Our late-time host-dominated spectrum indicates that the host galaxy likely harbors a weak active galactic nucleus. The initial Hubble Space Telescope (HST) spectrum of AT 2019qiz exhibits an iron and low-ionization broad absorption line (FeLoBAL) system that is seen for the first time in a TDE. This spectrum also bears a striking resemblance to that of Gaia16apd, a superluminous supernova. Our observations provide insights into the outflow properties in TDEs and show evidence for a connection between TDEs and engine-powered supernovae at early phases, as originally suggested by Metzger & Stone. In a time frame of 50 days, the UV spectra of AT 2019qiz started to resemble those of previous TDEs with only high-ionization broad absorption lines. The change in UV spectral signatures is accompanied by a decrease in the outflow velocity, which began at 15,000 km s-1 and decelerated to ∼10,000 km s-1. A similar evolution in the Hα emission-line width further supports the speculation that the broad Balmer emission lines are formed in TDE outflows. In addition, we detect narrow absorption features on top of the FeLoBAL signatures in the early HST UV spectrum of AT 2019qiz. The measured H i column density corresponds to a Lyman-limit system, whereas the metal absorption lines (such as N v, C iv, Fe ii, and Mg ii) are likely probing the circumnuclear gas and interstellar medium in the host galaxy.