Pair-dominated active galactic nuclei and the origin of the X-ray background

We discuss the possibility that the major energy contribution to the hard X-ray background (XRB) is the integrated emission from a mixed population of obscured and unobscured active galactic nuclei (AGN), as predicted by the Seyfert unification scheme. The model assumes that AGN are compact and in thermal pair equilibrium, and that the source compactness (i.e. the luminosity-to-size ratio) undergoes evolution following the X-ray luminosity evolution. Owing to the process of pair production, the model predicts evolution in the spectral properties, which are self-consistently taken into account. The XRB emission can be accounted for by a population with a relatively limited range of temperatures, around a few hundred keV. Finally, we estimate the number counts predicted in the LR band and the contribution of this population of AGN to the diffuse IR background, as a result of re-emission of the absorbed high-energy radiation.