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>~4 quasars known and hence represent ideal witnesses of the end of the ``dark age.'' Nine quasars are detected by Chandra, with ~2-57 counts in the observed 0.5-8 keV band. These detections increase the number of X-ray-detected AGNs at z>~4 to ~90 overall, Chandra has detected ~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 (<α_ox>=-1.88+/-0.05) is steeper than that typically found for z>~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 (~870 counts) is well parameterized by a power law with Γ=1.93^+0.10_-0.09 in the rest-frame ~2-40 keV band, and a tight upper limit of N_H~5×10²¹ 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.