logN–logSRelations and Spectral Properties of Sources from theASCALarge Sky Survey: Their Implications for the Origin of the Cosmic X‐Ray Background (CXB)

We carried out the first wide-area unbiased survey with the ASCA satellite inthe 0.7-10 keV band around a north Galactic-pole region covering a continuousarea of 7 square degrees (Large Sky Survey; LSS). To make the best use of ASCAcapability, we developed a new source-detection method where the complicateddetector responses are fully taken into account. Applying this method to theentire LSS data independently in the total (0.7-7 keV), hard (2-10 keV), andsoft (0.7-2 keV) band, we detected 107 sources altogether with sensitivitylimits of 6 x 10E-14 (0.7-7 keV), 1 x 10E-13 (2-10 keV), and 2 x 10E-14 ergsE-1 cmE-2 (0.7-2 keV), respectively. A complete list of the detected sourcesis presented. Based on detailed studies by Monte Carlo simulations, weevaluated effects of the source confusion and accurately derived Log N - Log Srelation in each survey band. The Log N - Log S relation in the hard band islocated on the extrapolation from the GINGA and HEAO1 results with theEuclidean slope of -3/2, while that in the soft band is consistent with theresults by ROSAT. At these flux limits, 30 (+/- 3) percent of the CXB in the0.7-7 keV band and 23 (+/- 3) percent in the 2-10 keV band have been resolvedinto discrete sources. The average spectrum of faint sources detected in thetotal band shows a photon index of 1.63 +/- 0.07 in the 0.7-10 keV range,consistent with the comparison of source counts between the hard and the softenergy band. Those detected in the hard band show a photon index of 1.49 +/-0.10 in the 2-10 keV range. These spectral properties suggest that contributionof sources with hard energy spectra become significant at a flux of 10E-13 ergsE-1 cmE-2 (2-10 keV). The most plausible candidates are type-II AGNs, asindicated by on-going optical identifications.Comment: 28 pages, 11 figures, to appear in ApJ 518, 1999; figure 1 replaced, minor errors in text correcte