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?

• Published in: arXiv.org
• Main Authors: Homan, D, Krumpe, M, Markowitz, A, Saha, T, Gokus, A, Partington, E, Lamer, G, Malyali, A, Liu, Z, Rau, A, Grotova, I, Cackett, E M, Buckley, D A H, Ciroi, S, F Di Mille, Gendreau, K, Gromadzki, M, Krishnan, S, Schramm, M, Steiner, J F
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 14.02.2023
• Subjects: Active galactic nuclei; Brightening; Deposition; Disruption; Emission; Galaxies; Gamma rays; Ignition; Ionization; Light curve; Luminosity; Photometry; Physics - High Energy Astrophysical Phenomena; Sky surveys (astronomy); Softness; X ray spectra; X-ray astronomy; XMM (spacecraft)
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2302.06989

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 \(1.76_{-0.24}^{+0.03} \times 10^{-11}\) erg cm\(^{-2}\) s\(^{-1}\). The X-ray ignition is associated with a galaxy at \(z=0.10\), making the peak luminosity log\(_{10}(L_{\rm 0.2-2keV}/[\textrm{erg s}^{-1}])\)=\(44.7\pm0.1\). 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. We present 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]\(\lambda\lambda\)4959,5007) and low-ionisation ([NII]\(\lambda\)6585, [SII]\(\lambda\lambda\)6716,6731) 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 \(\sim\)0.45 over 33 days and the UV shows a drop of \(\sim\)0.35. 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 \(\gamma\)-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 its classification. 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.