A ~3.8 hour Periodicity from an Ultrasoft Active Galactic Nucleus
  Candidate
  Very few galactic nuclei are found to show significant X-ray quasi-periodic
oscillations (QPOs). After carefully modeling the noise continuum, we find that
the ~3.8 hr QPO in the ultrasoft active galactic nucleus (AGN) candidate 2XMM
J123103.2+110648 was significantly detected (~5sigma) in two XMM-Newton
observations in 2005, but not in the one in 2003. The QPO rms is very high and
increases from ~25% in 0.2-0.5 keV to ~50% in 1-2 keV. The QPO probably
corresponds to the low-frequency type in Galactic black hole X-ray binaries,
considering its large rms and the probably low mass (~10^5 msun) of the black
hole in the nucleus. We also fit the soft X-ray spectra from the three
XMM-Newton observations and find that they can be described with either pure
thermal disk emission or optically thick low-temperature Comptonization. We see
no clear X-ray emission from the two Swift observations in 2013, indicating
lower source fluxes than those in XMM-Newton observations.
