Results from the ASCA Large Sky Survey — Nature of faint X-ray sources and the implications for the origin of the CXB Original Research Article

We report results from the ASCA Large Sky Survey (LSS), the first wide-area unbiased survey in the 0.7–10 keV band over a continuous region of 7 deg2 near the north Galactic pole. Applying the newly developed source-detection method to the LSS data independently in the total (0.7-7 keV), hard (2–10 keV), and soft (0.7-2 keV) band, we detected 107 sources altogether with sensitivity limits of 6 × 10−14 (0.7-7 keV), 1 × 10−13 (2–10 keV), and 2 × 10−14 erg s−1 cm−2 (0.7-2 keV), respectively. At these flux limits, 30(±3)% of the CXB in the 0.7-7 keV band and 23(±3)% in the 2–10 keV band have been resolved into discrete sources. The average spectrum of faint sources near the sensitivity limits show a photon index of 1.49±0.10 in the 2 10 keV range, and 1.63±0.07 in the 0.7–10 keV range. These spectral properties suggest that contribution of sources with hard energy spectra become significant at a flux level of 10−13 erg s−1 cm−2 (2–10 keV). Optical identification for a sample of 34 sources detected in the hard band revealed that at least six of them are type-2 AGNs. We found a good correlation that hard sources correspond to type-2 AGNs, while the photon index of type-1 AGNs is distributed around 1.6-1.8 in the 0.7–10 keV range. Follow-up X-ray observations of selected five hard-sources indicate that their spectra are subject to heavy absorption with an intrinsic hydrogen column density of 1022 ∼ 1023 cm−2. These results support the scenario that the CXB is the mixture of the emission from non-absorbed sources and absorbed sources.