Counts, Evolution, and Background Contribution of X-Ray Quasars and Other Extragalactic X-Ray Sources

The authors are presenting a study of the statistics of extragalactic X-ray sources, in particular quasars and active galactic nuclei of lower luminosity. First, they discuss the X-ray observations of optically selected quasars in the Bright Quasar Survey sample and in the faint Braccesi sample and review available samples of extragalactic X-ray sources for which optical identifications are essentially complete. The authors use evolution models for optically selected quasars to derive the evolution of X-ray quasars, as well as their source counts and background contribution. They employ essentially the same method to discuss AGNs of low luminosity, BL Lac objects, clusters of galaxies, and galaxies. Finally, the composition of the X-ray background and the spectral index of the unexplained component are discussed.