AT 2019avd: a novel addition to the diverse population of nuclear transients

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
e-mail: amalyali@mpe.mpg.de
² Department of Astronomy, University of Maryland, College Park, MD 20742, USA
³ Space Telescope Science Institute, Baltimore, MD 21218, USA
⁴ Department of Astronomy and the Oskar Klein Centre, Stockholm University, AlbaNova, SE 10691 Stockholm, Sweden
⁵ Institute for Astronomy, University of Hawaii at Manoa, 2680 Woodlawn Dr., Honolulu, HI 96822, USA
⁶ The School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
⁷ CIFAR Azrieli Global Scholars program, CIFAR, Toronto, Canada
⁸ Núcleo de Astronomía de la Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
⁹ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
¹⁰ Center for Cosmology and Particle Physics, New York University, NY 10003, USA
¹¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹² The Oskar Klein Centre, Department of Physics, AlbaNova, Stockholm University, SE 10691 Stockholm, Sweden
¹³ International Centre for Radio Astronomy Research – Curtin University, GPO Box U1987, Perth, WA 6845, Australia
¹⁴ INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo, 11, 34143 Trieste, Italy
¹⁵ Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
¹⁶ Las Cumbres Observatory, 6740 Cortona Dr, Suite 102, Goleta, CA 93117-5575, USA
¹⁷ Graduate School of Science and Engineering, Saitama Univ., 255 Shimo-Okubo, Sakura-ku, Saitama City, Saitama 338-8570, Japan
¹⁸ DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195, USA
¹⁹ Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
²⁰ IPAC, California Institute of Technology, 1200 E. California Boulevard, Pasadena, CA 91125, USA
²¹ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
²² Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
²³ California Institute of Technology, Pasadena, CA 91125, USA

Received: 15 October 2020
Accepted: 18 December 2020

Abstract

We report on SRG/eROSITA, ZTF, ASAS-SN, Las Cumbres, NEOWISE-R, and Swift XRT/UVOT observations of the unique ongoing event AT 2019avd, located in the nucleus of a previously inactive galaxy at z = 0.029. eROSITA first observed AT 2019avd on 2020-04-28 during its first all sky survey, when it was detected as an ultra-soft X-ray source (kT ~ 85 eV) that was ≳90 times brighter in the 0.2−2 keV band than a previous 3σ upper flux detection limit (with no archival X-ray detection at this position). The ZTF optical light curve in the ~450 days preceding the eROSITA detection is double peaked, and the eROSITA detection coincides with the rise of the second peak. Follow-up optical spectroscopy shows the emergence of a Bowen fluorescence feature and high-ionisation coronal lines ([Fe X] 6375 Å, [Fe XIV] 5303 Å), along with persistent broad Balmer emission lines (FWHM ~ 1400 km s⁻¹). Whilst the X-ray properties make AT 2019avd a promising tidal disruption event (TDE) candidate, the optical properties are atypical for optically selected TDEs. We discuss potential alternative origins that could explain the observed properties of AT 2019avd, such as a stellar binary TDE candidate, or a TDE involving a super massive black hole binary.