2017A&A...603A.128N

We present a systematic analysis of X-ray archival data of all the 29 quasars (QSOs) at z>5.5 observed so far with Chandra, XMM-Newton and Swift-XRT, including the most-distant quasar ever discovered, ULAS J1120+0641 (z=7.08). This study allows us to place constraints on the mean spectral properties of the primordial population of luminous Type 1 (unobscured) quasars. Eighteen quasars are detected in the X-ray band, and we provide spectral-fitting results for their X-ray properties, while for the others we provide upper limits to their soft (0.5-2.0keV) X-ray flux. We measured the power-law photon index and derived an upper limit to the column density for the five quasars (J1306+0356, J0100+2802, J1030+0524, J1148+5251, J1120+0641) with the best spectra (>30 net counts in the 0.5-7.0keV energy range) and find that they are consistent with values from the literature and lower-redshift quasars. By stacking the spectra of ten quasars detected by Chandra in the redshift range 5.7≥z≥6.1 we find a mean X-ray power-law photon index of Γ=1.92_–0.27^+0.28 and a neutral intrinsic absorption column density of N_H≥10²³cm^–2. These results suggest that the X-ray spectral properties of luminous quasars have not evolved up to z~=6. We also derived the optical-X-ray spectral slopes (α_ox) of our sample and combined them with those of previous works, confirming that α_ox strongly correlates with UV monochromatic luminosity at 2500Å. These results strengthen the non-evolutionary scenario for the spectral properties of luminous active galactic nuclei (AGN).