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

Creators: Hung, T.; Cenko, S. B.; Roth, Nathaniel; Gezari, S.; Veilleux, S.; van Velzen, Sjoert; Gaskell, C. Martin; Foley, Ryan J.; Blagorodnova, N.; Yan, Lin; Graham, M. J.; Brown, J. S.; Siebert, M. R.; Frederick, Sara; Ward, Charlotte; Gatkine, Pradip; Gal-Yam, Avishay; Yang, Yi; Schulze, S.; Dimitriadis, G.; Kupfer, Thomas; Shupe, David L.; Rusholme, Ben; Masci, Frank J.; Riddle, Reed; Soumagnac, Maayane T.; van Roestel, J.; Dekany, Richard

Abstract

We report the discovery of nonstellar hydrogen Balmer and metastable helium absorption lines accompanying a transient, high-velocity (0.05c) broad absorption line (BAL) system in the optical spectra of the tidal disruption event (TDE) AT2018zr (z = 0.071). In the Hubble Space Telescope UV spectra, absorption of high- and low-ionization lines is also present at this velocity, making AT2018zr resemble a low-ionization BALQSO. 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 Hα emission in a spherically expanding medium without invoking the typical prescription of an elliptical disk. We also report the appearance of narrow (~1000 kms^(−1)) N III λ4640, He II λ4686, Hα, and Hβ emission in the late-time optical spectra of AT2018zr, which may be a result of UV continuum hardening at late times, as observed by Swift. Including AT2018zr, we find a high association rate (three out of four) of BALs in the UV spectra of TDEs. This suggests that outflows may be ubiquitous among TDEs and less sensitive to viewing angle effects compared to QSO outflows.

Additional Information

© 2019 The American Astronomical Society. Received 2019 March 11; revised 2019 May 16; accepted 2019 May 26; published 2019 July 15. T.H. thanks Fred Hamann for sharing a copy of his draft prior to publication. T.H. is grateful to Jane Dai and Enrico Ramirez-Ruiz for helpful discussions. The authors would like to thank the anonymous referee for suggestions that greatly improved the clarity of the paper. The UCSC team is supported in part by NSF grant AST-1518052, the Gordon & Betty Moore Foundation, the Heising-Simons Foundation, and a fellowship from the David and Lucile Packard Foundation to R.J.F. S.V. acknowledges support from a Raymond and Beverley Sackler Distinguished Visitor Fellowship and thanks the host institute, the Institute of Astronomy, where this work was concluded. S.G is supported in part by NSF grant 1454816. S.V. also acknowledges support by the Science and Technology Facilities Council (STFC) and the Kavli Institute for Cosmology, Cambridge. N.B. acknowledges that this work is part of the research program VENI, with project No. 016.192.277, which is (partly) financed by the Netherlands Organisation for Scientific Research (NWO). M.R.S. is supported by the National Science Foundation Graduate Research Fellowship Program under grant No. 1842400. A.G.-Y. is supported by the EU via ERC grant No. 725161, the ISF, the BSF Transformative program, and a Kimmel award. The research of Y.Y. is supported through a Benoziyo Prize Postdoctoral Fellowship. Y.Y. thanks support astronomers at La Palma for assisting the WHT observation in service mode. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. The results made use of the Discovery Channel Telescope at Lowell Observatory. Lowell is a private, nonprofit institution dedicated to astrophysical research and public appreciation of astronomy and operates the DCT in partnership with Boston University, the University of Maryland, the University of Toledo, Northern Arizona University, and Yale University. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. The William Herschel Telescope is operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. The ACAM spectroscopy was obtained as part of OPT/2018A/017. The results in this work are based on observations obtained with the Samuel Oschin Telescope 48 inch and the 60 inch telescope at the Palomar Observatory as part of the Zwicky Transient Facility (ZTF) project. The ZTF is supported by the National Science Foundation under grant No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC, and UW. The SEDm is based upon work supported by the National Science Foundation under grant No. 1106171.

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