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 \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 4 quasars known and hence represent ideal witnesses of the end of the “ dark age ” .
Nine quasars are detected by Chandra , with \approx 2–57 counts in the observed 0.5–8 keV band .
These detections increase the number of X-ray detected AGN at z \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 4 to \approx 90 ; overall , Chandra has detected \approx 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 ( \langle \alpha _ { ox } \rangle = - 1.88 \pm { 0.05 } ) is steeper than that typically found for z \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 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 ( \approx 870 counts ) is well parameterized by a power law with \Gamma =1.93 ^ { +0.10 } _ { -0.09 } in the rest-frame \approx 2–40 keV band , and a tight upper limit of N _ { H } \approx 5 \times 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 zero and six , suggesting that the physical processes of accretion onto massive black holes have not changed over the bulk of cosmic time .