One of the main results of recent studies of quasar evolution is that the quasar luminosity function evolves strongly with redshift (e.g., Boyle et al. 1987, MNRAS, 227, 717). Many studies of quasar evolution are aimed at explaining this luminosity evolution. The X-ray band probes the innermost regions of the central engine of the AGN. The study of AGN in X-rays may possibly answer the question of whether there is an evolution in their central engines and how this is related to the evolution of the quasar luminosity function. It is also important to compare the properties of quasars near the peak of their comoving number density, thought to have occurred at z~2, with low redshift quasars. This comparison may provide clues to the cause of the dramatic decay of the quasar number density as the Universe expanded.
We recently presented results from a mini-survey of relatively high redshift (1.7 < z < 4) gravitationally lensed radio-quiet quasars observed with the Chandra X-ray Observatory and XMM-Newton. The lensing magnification effect allows us to search for changes in quasar spectroscopic and variability properties and X-ray with redshift over three orders of magnitude in intrinsic X-ray luminosity. We found a possible correlation between the X-ray power law photon index and X-ray luminosity of the gravitationally lensed radio-quiet quasar sample. The spectral slope becomes steeper as the X-ray luminosity increases. Such a correlation is not found in nearby z<0.1 quasars suggesting that quasars at redshifts near the peak of their number density may have different accretion properties than low redshift quasars. We suggest that this correlation can be understood in the context of the hot-corona model for X-ray emission from quasar accretion disks, under the hypothesis that quasars in our sample accrete very close to their Eddington limits and the observed luminosity range is set by the range of black hole masses. This hypothesis explains the observed correlation and is also consistent with recent predictions of semi-analytic models for quasar evolution.