Chandra and XMM-Newton observations of the first quasars:: X-rays from the age of cosmic enlightenment

We report on Chandra and XMM-Newton observations of a sample of 13 quasars at zapproximate to4.7-5.4 mostly taken from the Sloan Digital Sky Survey (SDSS). The present sample complements previous X-ray studies of zgreater than or equal to4 quasars, in which the majority of the objects are optically more luminous and at lower redshifts. All but two of our quasars have been detected in the X-ray band, thus doubling the number of zgreater than or equal to4.8 X-ray detected quasars. The two nondetections are likely to be due to a short exposure time (SDSSp J033829.31+002156.3) and to the presence of intrinsic absorption ( SDSSp J173744.87+582829.5). We confirm and extend to the highest redshifts the presence of a correlation between AB(1450)(1+z) magnitude and soft X-ray flux for zgreater than or equal to4 quasars and the presence of a steeper optical-to-X-ray spectral energy distribution (parameterized by alpha(ox)) for high-luminosity, high-redshift quasars than for lower luminosity, lower redshift quasars. The second effect is likely due to the known anticorrelation between alpha(ox) and rest-frame 2500 Angstrom luminosity, whose significance is confirmed via partial correlation analysis. The joint approximate to2.5-36 keV rest-frame spectrum of the z>4.8 SDSS quasars observed thus far by Chandra is well parameterized by a power law this photon index Gamma=1.84(-0.30)(+0.31) is consistent with those of zapproximate to0-3 quasars and that obtained from joint spectral fitting of zapproximate to4.1-4.5 optically luminous Palomar Digital Sky Survey quasars. No evidence for widespread intrinsic X-ray absorption has been found (N(H)less than or similar to4.0x10(22) cm(-2) on average at 90% confidence). We also obtained Hobby-Eberly Telescope (HET) photometric observations for eight of our target quasars. None of these shows significant (greater than 30%) optical variability over the time interval of a few years (in the observed frame) between the SDSS and HET observations.