Why do broad line AGN show up among faint hard X-ray sources?

Resumen del trabajo presentado a la Conferencia X-Ray Astronomy: Stellar Endpoints, AGN, and the Diffuse X-ray Background, celebrada en Bolonia (Italia) del 6 al 10 de septiembre de 1999.We have assembled a sample of ROSAT sources that are harder than the X-ray Background (XRB). By studying a hard X-ray spectrum population we are trying to discover the nature of the as yet unidentified source population needed to resolve the XRB spectral paradox. Current theory would suggest that narrow line QSOs (so called QSO2 objects) should be important, but we find that broad line AGN predominate. Here we discuss the nature of the broad line AGN and show that the absorption responsible for hardening the X-ray spectrum has a variety of optical signatures.Peer Reviewe