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 an ∼3.8 h 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 10 yr ago. There seemed to be significant spectral softening associated with the drop of X-ray flux (disc 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.