%0 Journal Article
%T High Energy Large Area Surveys: from BeppoSAX to Chandra and XMM
%A F. Fiore
%A A. Comastri
%A F. La Franca
%A C. Vignali
%A G. Matt
%A G. C. Perola
%J Physics
%D 2001
%I arXiv
%R 10.1007/10854354_73
%X Optical identification of hard X-ray selected BeppoSAX and Chandra sources indicates that a large fraction of the sources are ``intermediate'' AGN, i.e. type 1.8-1.9 AGN, broad-line quasars and even X-ray loud but optically dull (apparently normal) galaxies, all obscured in X-rays by columns of the order of logNH=22-23.5 cm-2. Because of this obscuring matter, these sources are more difficult to detect or select at other wavelengths, implying that a fraction of the accretion power in the Universe may have been missed so far.
%U http://arxiv.org/abs/astro-ph/0102041v1