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 d eg(2) of sky in the 5-10 keV band down to a flux of 4-5 x 10(-14) erg cm(-2 ) s(-1). The source surface density of 16.9 +/- 6.4 deg(-2) at the survey l imit corresponds to a resolved fraction of the 5-10 keV X-ray background (X RB) of the order of 20-30 per cent. Hardness ratio analysis indicates that the spectra of a substantial fractio n of the HELLAS sources (at least one-third) are harder than a alpha (E) = 0.6 power law. This hardness may be caused by 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, s uperimposed on a strongly cut-off power law, is likely to be present in sev eral sources.