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 _-0.09 ^+0.10 in the rest-frame ≈2-40 keV band, and a tight upper limit of N _H ≈ 5 × 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.