A candidate active galactic nucleus with a pure soft thermal X-ray spectrum

We report the discovery of a candidate active galactic nucleus (AGN), 2XMM J123103.2+110648 at z = 0.13, with an X-ray spectrum represented purely by soft thermal emission reminiscent of Galactic black hole (BH) binaries in the disk-dominated state. This object was found in the second XMM-Newton serendipitous source catalog as a highly variable X-ray source. In three separate observations, its X-ray spectrum can be represented either by a multicolor disk blackbody model with an inner temperature of kT _in 0.16-0.21keV or a Wien spectrum Comptonized by an optically thick plasma with kT 0.14-0.18keV. The soft X-ray luminosity in the 0.5-2keV band is estimated to be (1.6-3.8) × 10⁴²ergs^-1. Hard emission above 2keV is not detected. The ratio of the soft to hard emission is the strongest among AGNs observed thus far. Spectra selected in high/low-flux time intervals are examined in order to study spectral variability. In the second observation with the highest signal-to-noise ratio, the low-energy (below 0.7keV) spectral regime flattens when the flux is high, while the shape of the high-energy part (1-1.7keV) remains unchanged. This behavior is qualitatively consistent with being caused by strong Comptonization. Both the strong soft excess and spectral change consistent with Comptonization in the X-ray spectrum imply that the Eddington ratio is large, which requires a small BH mass (smaller than 10 ⁵ M).