Nanni, R.R.NanniVignali, C.C.VignaliGILLI, RobertoRobertoGILLIMORETTI, AlbertoAlbertoMORETTIBrandt, W. N.W. N.Brandt2020-09-032020-09-0320170004-6361http://hdl.handle.net/20.500.12386/27087We 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 (0.5-2.0 keV) 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 0.5-7.0 keV 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<SUB>-0.27</SUB><SUP>+0.28</SUP> and a neutral intrinsic absorption column density of N<SUB>H</SUB> ≤ 10<SUP>23</SUP> cm<SUP>-2</SUP>. 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<SUB>ox</SUB>) of our sample and combined them with those of previous works, confirming that α<SUB>ox</SUB> strongly correlates with UV monochromatic luminosity at 2500 Å. These results strengthen the non-evolutionary scenario for the spectral properties of luminous active galactic nuclei (AGN).STAMPAenArticle10.1051/0004-6361/2017304842-s2.0-85025461065000406619100098https://arxiv.org/abs/1704.08693https://www.aanda.org/articles/aa/abs/2017/07/aa30484-17/aa30484-17.html2017A&A...603A.128NFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation