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, XMMNewton 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 powerlaw photon index of λ = 1:92+0:28 -0:27 and a neutral intrinsic absorption column density of NH ≤ 1023 cm-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 (αox) of our sample and combined them with those of previous works, confirming that α ox strongly correlates with UV monochromatic luminosity at 2500 A. These results strengthen the non-evolutionary scenario for the spectral properties of luminous active galactic nuclei (AGN).
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science