A Medium Survey of the Hard X‐Ray Sky withASCA. II. The Source’s Broadband X‐Ray Spectral Properties

A complete sample of 60 serendipitous hard X-ray sources with flux in the range ~ 1 × 10⁻¹³ to ~ 4 × 10⁻¹² (2 - 10 keV), detected in 87 ASCA GIS2 images, was recently presented in literature. Using this sample it was possible to extend the description of the 2-10 keV LogN(>S)-LogS down to a flux limit of ~ 6× 10⁻¹⁴ (the faintest detectable flux), resolving about a quarter of the Cosmic X-ray Background. In this paper we have combined the ASCA GIS2 and GIS3 data of these sources to investigate their X-ray spectral properties using the "hardness" ratios and the "stacked" spectra method. Because of the sample statistical representativeness, the results presented here, that refer to the faintest hard X-ray sources that can be studied with the current instrumentation, are relevant to the understanding of the CXB and of the AGN unification scheme.