Testing the AGN models for the X-ray background
  We will briefly examine the following three issues related to the AGN synthesis models for X-ray background: 1) the possibility that absorbed AGNs evolve faster than unabsorbed ones; 2) the existence of the still debated population of luminous absorbed AGNs, the so-called QSO2s; 3) the behavior of the AGN density at a redshift above ~3. 
R. Gilli