Exploratory Chandra observations of the highest-redshift quasars: X-rays from the dawn of the modern universe

We report exploratory Chandra observations of 14 high-redshift (z = 4.06-5.27), optically selected quasars. Ten of these quasars are detected, increasing the number of z > 4 X-ray detected quasars by 71%. Our detections include four of the five highest-redshift X-ray detected quasars to date, among them SDSSp J021043.17-001818.4, the highest-redshift (z = 4.77) radio-loud quasar detected in the X-ray band. The four undetected objects are the broad absorption-line quasars SDSSp J112956.10-014212.4 and SDSSp J160501.21-011220.0, the weak emission-line quasar SDSSp J153259.96-003944.1, and the quasar PSS 1435 + 3057. A comparison of the quasars' spectral energy distributions (by means of the optical-to-X-ray spectral index α_ox) with those of lower-redshift samples indicates that the Chandra quasars are X-ray fainter by a factor of ≈2. X-ray faintness could be associated with the presence of large amounts of gas in the primeval galaxies harboring these high-redshift quasars, as suggested by recent studies conducted on z > 4 quasars in other bands. Using the current Chandra data, predictions for the next generation of X-ray observatories, Constellation-X and XEUS, are also provided.