Large Decay of X-ray Flux in 2XMM J123103.2+110648: Evidence for a Tidal
  Disruption Event
  The X-ray source 2XMM J123103.2+110648 was previously found to show pure
thermal X-ray spectra and a ~3.8 hr periodicity in three XMM-Newton X-ray
observations in 2003-2005, and the optical spectrum of the host galaxy
suggested it as a type 2 active galactic nucleus candidate. We have obtained
new X-ray observations of the source, with Swift and Chandra in 2013-2016, in
order to shed new light on its nature based on its long-term evolution
property. We found that the source could be in an X-ray outburst, with the
X-ray flux decreasing by an order of magnitude in the Swift and Chandra
observations, compared with the XMM-Newton observations ten years ago. There
seemed to be significant spectral softening associated with the drop of X-ray
flux (disk temperature kT ~ 0.16-0.2 keV in XMM-Newton observations versus
kT~0.09+-0.02 keV in the Chandra observation. Therefore the Swift and Chandra
follow-up observations support our previous suggestion that the source could be
a tidal disruption event (TDE), though it seems to evolve slower than most of
the other TDE candidates. The apparent long duration of this event could be due
to the presence of a long super-Eddington accretion phase and/or slow
circularization.
