The BeppoSAX High Energy Large Area Survey HELLAS, II: Number counts and X-ray spectral properties

The BeppoSAX High Energy Large Area Survey (HELLAS) has surveyed about 85 deg of sky in the 5–10 keV band down to a flux of 4− 5× 10 erg cm s . The source surface density of 16.9±6.4 deg at the survey limit corresponds to a resolved fraction of the 5–10 keV X–ray background (XRB) of the order of 20–30 %. Hardness ratios analysis indicates that the spectra of a substantial fraction of the HELLAS sources (at least one third) are harder than a αE = 0.6 power law. This hardness may be due to large absorbing columns. The hardness ratio analysis also indicates that many HELLAS sources may have a spectrum more complex than a single absorbed power law. A soft component, superimposed to a strongly cut-off power law, is likely to be present in several sources.