A Study of Quasar Evolution in the X‐Ray Band with the Aid of Gravitational Lensing

We present results from a mini-survey of relatively high redshift (1.7 < z <4) gravitationally lensed radio-quiet quasars observed with the Chandra X-rayObservatory and with XMM-Newton. The lensing magnification effect allows us tosearch for changes in quasar spectroscopic and flux variability properties withredshift over three orders of magnitude in intrinsic X-ray luminosity. Itextends the study of quasar properties to unlensed X-ray flux levels as low asa few times 10^{-15} erg cm^{-2} s^{-1} in the observed 0.4-8 keV band. We finda possible correlation between the X-ray powerlaw photon index and X-rayluminosity of the gravitationally lensed radio-quiet quasar sample. The X-rayspectral slope steepens as the X-ray luminosity increases. This correlation isstill significant when we combine our data with other samples of radio-quietquasars with z > 1.5, especially in the low luminosity range between10^{43}-10^{45.5} erg s^{-1}. This result is surprising considering that such acorrelation is not found for quasars with redshifts below 1.5. We suggest thatthis correlation can be understood in the context of the hot-corona model forX-ray emission from quasar accretion disks, under the hypothesis that thequasars in our sample accrete very close to their Eddington limits and theobserved luminosity range is set by the range of black hole masses (thishypothesis is consistent with recent predictions of semi-analytic models forquasar evolution). The upper limits of X-ray variability of our relatively highredshift sample of lensed quasars are consistent with the known correlationbetween variability and luminosity observed in Seyfert 1s when this correlationis extrapolated to the larger luminosities of our sample.Comment: 21 pages, including 6 tables, 7 figures, accepted by Ap