The BeppoSAX High Energy Large Area Survey (HELLAS) has surveyed about 85deg2 of sky in the 5-10keV band down to a flux of 4-5x10-14erg/cm2/s. The source surface density of 16.9+/-6.4deg2 at the survey limit corresponds to a resolved fraction of the 5-10keV X-ray background (XRB) of the order of 20-30 per cent. Hardness ratio 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 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, superimposed on a strongly cut-off power law, is likely to be present in several sources.
Fiore, F., Giommi, P., Vignali, C., Comastri, A., Matt, G., Perola G., C., et al. (2002). BeppoSAX HELLAS survey. II.