Unconventional AGN in hard X-ray surveys
  Extensive programs of follow-up observations of hard X-ray selected sources have unambiguosly revealed that the sources of the X-ray background are characterized by an extremely large dispersion in their optical magnitude and spectroscopic classification. Here we present the results of an attempt to understand the nature of the observed variety using a simple prescription for their optical to X-ray energy distribution. 
A. Comastri (1), M. Brusa (2), M. Mignoli (1) and the HELLAS2XMM   collaboration ((1) INAF - OA Bologna, (2) Dipartimento di...