Quasar evolution and luminosity function - An X-ray perspective

X-ray properties of optically selected QSOs are reviewed, and their implications for the QSO evolution and luminosity function are summarized. Analyses of the explicit dependence of the X-ray to optical luminosity ratio parameter alpha (a,x) on redshift z and optical luminosity Lo are described, and results from a recent analysis of complete, magnitude-limited samples of optically selected QSOs are presented. It is shown that probably all optically-selected QSOs are X-ray loud and that comparisons of the optical evolution and luminosity function, combined with the alpha(o,x)(z,Lo) dependence, with X-ray-selected samples are numerically sensitive to the details of the input.