The X-ray background and the deep X-ray surveys Original Research Article

The deep X-ray surveys performed by the two major X-ray observatories on flight, Chandra and XMM, are being resolving the bulk of the cosmic X-ray background (XRB) in the 2–10 keV energy band, where the sky flux is dominated by extragalactic emission. Although the actual fraction depends on the absolute sky flux, which is measured with an uncertainty of ∼40%, most of the XRB is already resolved. Optical identifications of the X-ray sources in the deep surveys are being showing that these are mainly AGN, most of which being obscured as predicted by population synthesis models. However, first results indicate that the redshift distribution of the sources making the XRB seems to peak at much lower redshift than predicted by the models. In this article, I will briefly review and discuss the measurements of the XRB spectrum and the AGN synthesis models of the XRB. Then, I will introduce the Chandra and XMM deep X-ray surveys, mainly focusing on the Chandra Deep Field North and South. Finally, the properties of the X-ray sources populating the deep surveys will be described and compared with the predictions of the most recent synthesis models.