On the nature of X-ray Bright Optically Normal galaxies
  Multiwavelength observations of the hard X-ray selected sources by Chandra and XMM-Newton surveys have significantly improved our knowledge of the objects responsible of the hard X-ray background. A surprising finding is the discovery of a population of optically dull, X-ray bright galaxies emerging at 2-10 keV fluxes of the order of 10^{-14} erg cm^{-2} s^{-1}. We present preliminary results of multiwavelength observations of a few objects serendipitously discovered in the field of XMM-Newton and Chandra observations and intensively studied at longer wavelengths. 
A. Comastri (Oss. Astronomico-Bologna), M. Brusa (Dip. di   Astronomia-Bologna), P. Ciliegi, M. Mignoli (Oss. Astronomico-Bo...