A Medium Survey of the Hard X‐Ray Sky with theASCAGas Imaging Spectrometer: The (2–10 keV) Number Counts Relationship

In this paper, we report the first results of a medium survey program conducted in the 2-10 keV energy band using data from the GIS2 instrument onboard the ASCA satellite. We have selected from the ASCA public archive (as of 1996 February 14) 87 images suitable for this project. Sixty serendipitous X-ray sources with a signal-to-noise ratio greater than 3.5 were found. The 2-10 keV flux of the detected sources ranges from ~1.1 × 10-13 ergs cm-2 s-1 to ~4.1 × 10-12 ergs cm-2 s-1. Using this sample, we have extended the description of the 2-10 keV log N(>S)-log S to a flux limit of ~6.3 × 10-14 ergs cm-2 s-1 (the faintest detectable flux), i.e., about 2.7 orders of magnitude fainter than the Piccinotti et al. determination. The derived number-flux relationship is well described by a power-law model, N(>S) = K × S-α, with best-fit values α = 1.67 ± 0.18 and K = 2.85 × 10-21 deg-2. At the flux limit of the survey, about 27% of the cosmic X-ray background in the 2-10 keV energy band is resolved in discrete sources. A flattening of the number-flux relationship, within a factor of 10 from the flux limit of the present survey, is expected in order to avoid saturation. The implications of these results for models of the origin of the hard X-ray background are briefly discussed.