Active galactic nuclei and star-forming galaxies – contributing to the extragalactic X-ray flux

The nature of discrete sources contributing to the extragalactic X-ray background (XRB) in the energy range ∼0.1-3 keV has been investigated via optical identifications based on positional coincidence and spectroscopy. The results presented here address the counterparts to the discrete X-ray sources detected in the Einstein Observatory deep X-ray survey in Pavo. X-ray selected quasars, detected to a limiting flux of ∼10 -14 erg cm -2 s -1 and a limiting B J magnitude of 21.5, have a surface density of ∼32 per square degree, an average redshift of 0.94, and account directly for ∼15 per cent of the extragalactic X-ray background at an effective energy of 2 keV