Discovery of an Ultrasoft X-ray Transient Source in the 2XMM Catalog: a
  Tidal Disruption Event Candidate
  We have discovered an ultrasoft X-ray transient source, 2XMMi
J184725.1-631724, which was detected serendipitously in two XMM-Newton
observations in the direction of the center of the galaxy IC 4765-f01-1504 at a
redshift of 0.0353. These two observations were separated by 211 days, with the
0.2-10 keV absorbed flux increasing by a factor of about 9. Their spectra are
best described by a model dominated by a thermal disk or a single-temperature
blackbody component (contributing >80% of the flux) plus a weak power-law
component. The thermal emission has a temperature of a few tens of eV, and the
weak power-law component has a photon index of ~3.5. Similar to the black hole
X-ray binaries in the thermal state, our source exhibits an accretion disk
whose luminosity appears to follow the $L\propto T^4$ relation. This would
indicate that the black hole mass is about 10^5-10^6 M_sun using the
best-fitting inner disk radius. Both XMM-Newton observations show variability
of about 21% on timescales of hours, which can be explained as due to fast
variations in the mass accretion rate. The source was not detected by ROSAT in
an observation in 1992, indicating a variability factor of >64 over longer
timescales. The source was not detected again in X-rays in a Swift observation
in 2011 February, implying a flux decrease by a factor of >12 since the last
XMM-Newton observation. The transient nature, in addition to the extreme
softness of the X-ray spectra and the inactivity of the galaxy implied by the
lack of strong optical emission lines, makes it a candidate tidal disruption
event. If this is the case, the first XMM-Newton observation would have been in
the rising phase, and the second one in the decay phase.
