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

Abstract

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.