Optical Identification of the Hardest X‐Ray Source in theASCALarge Sky Survey

We report the optical identification of the hardest X-ray source (AXJ131501+3141) detected in an unbiased wide-area survey in the 0.5--10 keV band,the ASCA Large Sky Survey. The X-ray spectrum of the source is very hard and iswell reproduced by a power-law component (Gamma = 1.5^+0.7_-0.6) with N_H =6^+4_-2 *10^22 cm^-2 (Sakano et al. 1998). We have found a galaxy with R=15.62mag near the center of the error circle for the X-ray source. The opticalspectrum of the galaxy shows only narrow emission lines whose ratios correspondto those of a type 2 Seyfert galaxy at z = 0.072, implying anabsorption-corrected X-ray luminosity of 2*10^43 erg sec^-1 (2--10 keV) and M_B= -20.93 mag. A radio point source is also associated with the center of thegalaxy. We thus identify the X-ray source with this galaxy as an obscured AGN.The hidden nature of the nucleus of the galaxy in the optical band isconsistent with the X-ray spectrum. These results support the idea that theobscured AGNs/QSOs contribute significantly to the cosmic X-ray background inthe hard band at the faint flux level.Comment: 20 pages with 5 postscript figures, uses aaspp4.sty, Ap.J. Accepte