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

Abstract

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. The X-ray spectrum of the source is very hard and is well 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=15.62 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*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 the galaxy. 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 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.

arXiv comment20 pages with 5 postscript figures, uses aaspp4.sty, Ap.J. Accepted

References in corpus (3)

• X-ray QSO evolution from a very deep ROSAT survey
• A deep ROSAT Survey X: X-ray Luminous Narrow Emission Line Galaxies
• AXJ1749+684: a narrow emission-line galaxy with a flat X-ray spectrum

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• A Medium Survey of the Hard X-Ray Sky with ASCA. II.: The Source's Broad Band X-Ray Spectral Properties
• Obscured and unobscured AGN populations in a hard-X-ray selected sample of the XMDS survey
• A candidate of a type-2 QSO at z=0.9: Large X-ray absorption with a strong broad-H-alpha emission line
• ASCA observations of deep ROSAT fields IV.IR and hard X-ray observations of an obscured high-redshift QSO
• The hardest X-ray source in the ASCA Large Sky Survey: Discovery of a new type 2 Seyfert