AGN X-ray absorption vs. optical classification: Hints from the XRB models

We present a synthesis model of the cosmic X-ray background (XRB) with Active Galactic Nuclei (AGNs) where the evolution of absorbed AGNs is assumed to be faster than that of unabsorbed AGNs and to stop at a lower redshift. The model provides a good description of the XRB spectrum over the broadband 1-100 keV and is in agreement with the AGN counts in the three different bands 0.5-2 keV, 2-10 keV and 5-10 keV, covered by recent X-ray surveys. We also find a relation between optical classification and X-ray absorption of AGNs, which reproduces the fractions of the different optical types observed in the X-ray surveys. As a result of our investigation, among the AGNs optically classified as type 1, the fraction of objects with X-ray absorbed spectra seems to increase with the redshift.