Astronomy and Astrophysics – Astrophysics
Scientific paper
2000-01-04
Astronomy and Astrophysics
Astrophysics
Invited talk, to appear in the Proceedings of the Conference: ``Large Scale Structure in the X-ray Universe" Santorini, Greece
Scientific paper
The integrated emission of highly obscured AGN is expected to provide a major contribution to the X-ray energy density in the Universe: the X-ray background (XRB). The study of these objects is possible only at energies where the effects of absorption are less severe. For this reason we have carried out the BeppoSAX High Energy LLarge Area Survey in the hardest band (5-10 keV) accessible so far with imaging X-ray instruments. The source surface density at the survey limiting flux accounts for a significant fraction (20-30%) of the hard XRB. The X-ray data complemented by multiwavelength follow--up observations suggest that a large fraction of the hard sources are AGN and that X-ray absorption with column densities in the range 10^22-23.5 cm-2 is common among them. The great diversity in their optical-near-IR properties suggests that the optical appearance of obscured sources is a function of the X-ray luminosity. We briefly discuss the implications of these findings for the XRB models.
Comastri Andrea
Fiore Fabrizio
La Franca Fabio
Matt Giorgia
Vignali Christian
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