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 z ≈ 4.7-5.4 mostly taken from the Sloan Digital Sky Survey (SDSS). The present sample complements previous X-ray studies of z ≥ 4 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 z ≥ 4.8 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 z ≥ 4 quasars and the presence of a steeper optical-to-X-ray spectral energy distribution (parameterized by α_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 α_ox and rest-frame 2500 Å luminosity, whose significance is confirmed via partial correlation analysis. The joint ≈2.5-36 keV rest-frame spectrum of the z > 4.8 SDSS quasars observed thus far by Chandra is well parameterized by a power law with photon index Γ = 1.84_-0.30^+0.31; this photon index is consistent with those of z ≈ 0-3 quasars and that obtained from joint spectral fitting of z ≈ 4.1-4.5 optically luminous Palomar Digital Sky Survey quasars. No evidence for widespread intrinsic X-ray absorption has been found (N_H ≲ 4.0 × 10²² 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.