An Ultrasoft X-ray Flare from 3XMM J152130.7+074916: a Tidal Disruption
  Event Candidate
  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 d_L=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
~5e43 erg/s, 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 yrs 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.
