Abstract
We present first results from the ASCA Medium-Sensitivity Survey (or the GIS catalog project). From the serendipitous fields amounting to 106 deg2, we determined the log N-log S relations in the 0.7-7 and 2-10 keV bands with the best statistical accuracy obtained so far, over the flux range from 1 × 10-11 to 5 × 10-14 and 1 × 10-11 to 7 × 10-14 ergs s-1 cm-2, respectively. When the sources detected in the 0.7-7 keV band are divided into two subsamples with higher and lower spectral hardness, the former exhibits a significantly steeper slope than the latter at fluxes below ~10-12 ergs s-1 cm-2 (0.7-7 keV). The average spectrum of sources becomes continuously harder toward fainter fluxes, from a photon index of 2.1 in the 0.7-10 keV range at the flux of ~10-11 to 1.6 at ~10-13 ergs s-1 cm-2 (0.7-7 keV). This is consistent with the comparison of source counts between the 2-10 and the 0.7-2 keV bands and solves the puzzle of their discrepancy reported previously. Our results demonstrate rapid emergence of hard X-ray sources with a decreasing flux from ~10-12 to ~10-13 ergs s-1 cm-2 (2-10 keV).
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