The ASCA Medium-Sensitivity Survey (the GIS catalog project):: Source counts and evidence for emerging population of hard sources

We present first results from the ASCA Medium-Sensitivity Survey (or the GIS catalog project). From the serendipitous fields amounting to 106 deg(2), 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 x 10(-11) to 5 x 10(-14) and 1 x 10(-11) to 7 x 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 similar to 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 similar to 10(-11) to 1.6 at similar to 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 similar to 10(-12) to similar to 10(-13) ergs s(-1) cm(-2) (2-10 keV).