The X-ray background and the evolution of AGN

We discuss the constraints on the AGN evolution from the cosmic X-ray background and source counts. A synthesis model to fit the X-ray background is presented. The data used are a compilation from Gruber (1992), mainly based on HEAO-1 data. The spectrum of type 2 AGN has been modeled including Compton down-scattering within the absorbing material. We found an improvement when a dependence on redshift of the relative number of obscured sources is introduced. In particular, we found a decrease of the fraction of type 2 AGN at redshifts larger than similar to 2. However, only the soft X-ray counts are well reproduced, while the hard (5-10 keV) BeppoSAX/HELLAS source counts are underestimated by a factor of similar to 1.5. A satisfactory global solution, including the good reproduction of the hard counts, is instead obtained if the XRB normalization, recently obtained from BeppoSAX/MECS measurements (about 30% higher than HEAO-1), is adopted.