Discovery of the luminous X-ray ignition eRASSt J234402.9−352640

I. Tidal disruption event or a rapid increase in accretion in an active galactic nucleus?^⋆

¹ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: dhoman@aip.de
² Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warszawa, Poland
³ University of California, San Diego, Center for Astrophysics and Space Sciences, MC 0424, La Jolla, CA 92093-0424, USA
⁴ Department of Physics, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA
⁵ Dr. Karl Remeis-Observatory & ECAP, Friedrich-Alexander-Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
⁶ Lehrstuhl für Astronomie, Universität Würzburg, Emil-Fischer-Straße 31, 97074 Würzburg, Germany
⁷ Wayne State University, Department of Physics & Astronomy, 666 W. Hancock St, Detroit, MI 48201, USA
⁸ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße, 85748 Garching, Germany
⁹ South African Astronomical Observatory, PO Box 9 Observatory, Cape Town 7935, South Africa
¹⁰ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
¹¹ Department of Physics, University of the Free State, PO Box 339 Bloemfontein 9300, South Africa
¹² Department of Physics and Astronomy, University of Padova, Via F. Marzolo 8, 35131 Padova, Italy
¹³ Las Campanas Observatory – Carnegie Institution for Science, Colina el Pino, Casilla 601, La Serena, Chile
¹⁴ Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁵ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
¹⁶ Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama City, Saitama 338-8570, Japan
¹⁷ Center for Astrophysics, Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA

Received: 28 September 2022
Accepted: 8 February 2023

Abstract

In November 2020, a new, bright object, eRASSt J234402.9−352640, was discovered in the second all-sky survey of SRG/eROSITA. The object brightened by a factor of at least 150 in 0.2–2.0 keV flux compared to an upper limit found six months previous, reaching an observed peak of 1.76_−0.24^+0.03 × 10⁻¹¹ erg cm⁻² s⁻¹. The X-ray ignition is associated with a galaxy at z = 0.10, making the peak luminosity log₁₀(L_{0.2−2 keV}/[erg s⁻¹]) = 44.7 ± 0.1. Around the time of the rise in X-ray flux, the nucleus of the galaxy brightened by approximately 3 mag. in optical photometry, after correcting for the host contribution. We present X-ray follow-up data from Swift, XMM-Newton, and NICER, which reveal a very soft spectrum as well as strong 0.2–2.0 keV flux variability on multiple timescales. Optical spectra taken in the weeks after the ignition event show a blue continuum with broad, asymmetric Balmer emission lines, and high-ionisation ([OIII]λλ4959,5007) and low-ionisation ([NII]λ6585, [SII]λλ6716,6731) narrow emission lines. Following the peak in the optical light curve, the X-ray, UV, and optical photometry all show a rapid decline. The X-ray light curve shows a decrease in luminosity of ∼0.45 over 33 days and the UV shows a drop of ∼0.35 over the same period. eRASSt J234402.9−352640 also shows a brightening in the mid-infrared, likely powered by a dust echo of the luminous ignition. We find no evidence in Fermi-LAT γ-ray data for jet-like emission. The event displays characteristics of a tidal disruption event (TDE) as well as of an active galactic nucleus (AGN), complicating the classification of this transient. Based on the softness of the X-ray spectrum, the presence of high-ionisation optical emission lines, and the likely infrared echo, we find that a TDE within a turned-off AGN best matches our observations.