The colours of the X-ray background
  The recent deep X-ray surveys at both soft (0.5--2 keV) and hard (2--10 keV) energies have greatly extended our knowledge of the X-ray source density and spectral shapes at relatively faint fluxes adding further evidence on the fact that discrete sources, mainly AGNs, are responsible for the X-ray background (XRB) emission over a broad energy range. In addition the first complete optically identified samples of soft X-ray sources are becoming available allowing to test the XRB AGN synthesis models in the light of recent results. In this paper I will briefly compare the model predictions with some new observational data. 
A. Comastri