Optical Identification of the Hardest X-Ray Source in the ASCA Large Sky Survey

We report the optical identification of the hardest X-ray source (AX J131501+3141) detected in an unbiased wide-area survey in the 0.5-10 keV band, the ASCA Large Sky Survey (LSS). The X-ray spectrum of the source is very hard and is well reproduced by a power-law component (Γ=1.5−0.6+0.7) with NH=6−2+4×1022 cm-2. We have found a galaxy with R = 15.62 mag near the center of the error circle for the X-ray source. The optical spectrum of the galaxy shows only narrow emission lines whose ratios correspond to those of a type 2 Seyfert galaxy at z = 0.072, implying an absorption-corrected X-ray luminosity of 2 × 1043 ergs s-1 (2-10 keV) and MB = -20.93 mag. A radio point source is also associated with the center of the galaxy. We thus identify the X-ray source with this galaxy as an obscured active galactic nucleus (AGN). The hidden nature of the nucleus of the galaxy in the optical band is consistent with the X-ray spectrum. These results support the idea that the obscured AGNs/QSOs contribute significantly to the cosmic X-ray background in the hard band at the faint flux level.