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- Title
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.
- 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.; Di Mille, F.; Gendreau, K.; Gromadzki, M.; Krishnan, S.; Schramm, M.; Steiner, J. F.
- Abstract
In November 2020, a new, bright object, eRASSt J234402.9352640, 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+0:03 0:24 1011 erg cm2 s1. The X-ray ignition is associated with a galaxy at z = 0:10, making the peak luminosity log10(L0:2 keV=[ergs]) = 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]--4959,5007) and low-ionisation ([NII]-6585, [SII]--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 ~0.45 over 33 days and the UV shows a drop of ~0.35 over the same period. eRASSt J234402.9352640 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 -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.
- Subjects
ACTIVE galactic nuclei; ECHO; X-rays; X-ray spectra; SEYFERT galaxies; OPTICAL spectra; LIGHT curves; INERTIAL confinement fusion
- Publication
Astronomy & Astrophysics / Astronomie et Astrophysique, 2023, Vol 672, p1
- ISSN
0004-6361
- Publication type
Article
- DOI
10.1051/0004-6361/202245078