AN ULTRASOFT X-RAY FLARE FROM 3XMM J152130.7+074916: A TIDAL DISRUPTION EVENT CANDIDATE

• Published in: The Astrophysical journal Vol. 811; no. 1; pp. 43 - 9
• Main Authors: Lin, Dacheng, Maksym, Peter W., Irwin, Jimmy A., Komossa, S., Webb, Natalie A., Godet, Olivier, Barret, Didier, Grupe, Dirk, Gwyn, Stephen D. J.
• Format: Journal Article
• Language: English
• Published: United States The American Astronomical Society 20.09.2015
• Subjects: ACCRETION DISKS; accretion, accretion disks; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BLACK HOLES; DETECTION; Disks; Disruption; Flux; Galaxies; galaxies: individual (3XMM J152130.7+074916); galaxies: nuclei; GALAXY NUCLEI; KEV RANGE; LUMINOSITY; MASS; STARS; TRANSIENTS; X-RAY GALAXIES; X-ray sources; X-RAY SPECTRA; X-rays; X-rays: galaxies; XMM (spacecraft)
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.1088/0004-637X/811/1/43

ABSTRACT We report on the discovery of an ultrasoft X-ray transient source, 3XMM J152130.7+074916. It was serendipitously detected in an XMM-Newton observation on 2000 August 23, and its location is consistent with the center of the galaxy SDSS J152130.72+074916.5 (z = 0.17901 and dL = 866 Mpc). The high-quality X-ray spectrum can be fitted with a thermal disk with an apparent inner disk temperature of 0.17 keV and a rest-frame 0.24-11.8 keV unabsorbed luminosity of ∼5 × 1043 erg s−1, subject to a fast-moving warm absorber. Short-term variability was also clearly observed, with the spectrum being softer at lower flux. The source was covered but not detected in a Chandra observation on 2000 April 3, a Swift observation on 2005 September 10, and a second XMM-Newton observation on 2014 January 19, implying a large variability (>260) of the X-ray flux. The optical spectrum of the candidate host galaxy, taken ∼11 years after the XMM-Newton detection, shows no sign of nuclear activity. This, combined with its transient and ultrasoft properties, leads us to explain the source as tidal disruption of a star by the supermassive black hole in the galactic center. We attribute the fast-moving warm absorber detected in the first XMM-Newton observation to the super-Eddington outflow associated with the event and the short-term variability to a disk instability that caused fast change of the inner disk radius at a constant mass accretion rate.