Hard sources, their colors, and the X-ray background

Trabajo presentado a la Conference on X-Ray Astronomy: "Stellar Endpoints, AGN, and the Diffuse X-ray Background", celebrada en Bologna (Italia) del 6 al 10 de septiembre de 1999.The unresolved soft and hard X-ray backgrounds (XRB) are produced by hard extragalactic sources of unknown nature. We have assembled a sample of hard sources in Rosat pointings that offer a preview of the properties of these sources. We present here optical and infrared photometry of both unidentified and identified objects in this sample. Our data provide evidence for absorption as the origin of the hard spectra of these sources. Furthermore, the unidentified sources have the same optical and X-ray colors as the identified ones, so they are probably the same population. Since most of the extragalactic identified sources in our sample are broad line AGN, our survey supports the idea of obscured AGN as the main sources of the (soft) XRB.Peer Reviewe