X-ray lighthouses of the high-redshift universe.: II.: Further snapshot observations of the most luminous z ≳ 4 quasars with Chandra

We report on Chandra observations of a sample of 11 optically luminous (M-B < - 28.5) quasars at z = 3.96- 4: 55 selected from the Palomar Digital Sky Survey and the Automatic Plate Measuring Facility Survey. These are among the most luminous z greater than or similar to 4 quasars known and hence represent ideal witnesses of the end of the dark age.'' Nine quasars are detected by Chandra, with approximate to 2 - 57 counts in the observed 0.5 - 8 keV band. These detections increase the number of X-ray-detected AGNs at z greater than or similar to 4 to approximate to 90; overall, Chandra has detected approximate to 85% of the high-redshift quasars observed with snapshot ( few kilosecond) observations. PSS 1506 +/- 5220, one of the two X-ray-undetected quasars, displays a number of notable features in its rest-frame ultraviolet spectrum, the most prominent being broad, deep Si IV and C IV absorption lines. The average optical-to-X-ray spectral index for the present sample ([alpha(ox)] = - 1.88 +/- 0.05) is steeper than that typically found for z greater than or similar to 4 quasars but consistent with the expected value from the known dependence of this spectral index on quasar luminosity. We present joint X-ray spectral fitting for a sample of 48 radio-quiet quasars in the redshift range 3.99 - 6.28 for which Chandra observations are available. The X-ray spectrum ( approximate to 870 counts) is well parameterized by a power law with Gamma = 1.93(-0.09)(+0.10) in the rest- frame approximate to 2-40 keV band, and a tight upper limit of N-H approximate to 5 x 10(21) cm(-2) is obtained on any average intrinsic X-ray absorption. There is no indication of any significant evolution in the X-ray properties of quasars between redshifts 0 and 6, suggesting that the physical processes of accretion onto massive black holes have not changed over the bulk of cosmic time.