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.
Categories: astro-ph