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?

Homan D, Krumpe M, Markowitz A, Saha T, Gokus A, Partington E, Lamer G, Malyali A, Liu Z, Rau A, Grotova I, Cackett EM, Buckley DA, Ciroi S, Di Mille F, Gendreau K, Gromadzki M, Krishnan S, Schramm M, Steiner JF (2023)

Publication Type: Journal article

Publication year: 2023

Journal

Astronomy & Astrophysics EDP Sciences

Book Volume: 672

Article Number: A167

DOI: 10.1051/0004-6361/202245078

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(L0.2-2keV/[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.

Involved external institutions

Leibniz Institute for Astrophysics Potsdam / Leibniz-Institut für Astrophysik Potsdam DE Germany (DE) Nicolaus Copernicus Astronomical Centre of the Polish Academy of Sciences PL Poland (PL) Washington University US United States (USA) (US) Wayne State University (WSU) US United States (USA) (US) Max-Planck-Institut für extraterrestrische Physik (MPE) / Max Planck Institute for Extraterrestrial Physics DE Germany (DE) South African Astronomical Observatory (SAAO) ZA South Africa (ZA) University of Padua / Universita degli Studi di Padova IT Italy (IT) National Aeronautics and Space Administration (NASA) US United States (USA) (US) University of Warsaw / Uniwersytet Warszawski PL Poland (PL) Saitama University / 埼玉大学 JP Japan (JP) Las Campanas Observatory (LCO) CL Chile (CL) Smithsonian Institution US United States (USA) (US)

How to cite

APA:

Homan, D., Krumpe, M., Markowitz, A., Saha, T., Gokus, A., Partington, E.,... Steiner, J.F. (2023). 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? Astronomy & Astrophysics, 672. https://dx.doi.org/10.1051/0004-6361/202245078

MLA:

Homan, D., et al. "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?" Astronomy & Astrophysics 672 (2023).

BibTeX: Download