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

Y. Ueda, M. Akiyama, G. Hasinger, H. Inoue, Y. Ishisaki, K. Makishima, Y. Ogasaka, K. Ohta, M. Sakano, T. Takahashi, T. Tsuru, W. Voges, T. Yamada

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3 Citations (Scopus)


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 deg 2 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 10 22 ∼ 10 23 cm -2 . These results support the scenario that the CXB is the mixture of the emission from non-absorbed sources and absorbed sources.

Original languageEnglish
Pages (from-to)839-844
Number of pages6
JournalAdvances in Space Research
Issue number3-4
Publication statusPublished - 2000


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