2020MNRAS.491.3884P

We present XMM-Newton X-ray observations and analysis of three Dark Energy Survey z > 6.5 quasars (VDES J0020-3653 at z = 6.824, VDES J0244-5008 at z = 6.724, and VDES J0224-4711 at z = 6.526) and six other quasars with 6.438 < z < 6.747 from the XMM-Newton public archive. Two of the nine quasars are detected at a high (>4σ) significance level: VDES J0224-4711(z = 6.53) at 9σ and PSO J159-02 (z = 6.38) at 8σ. They have a photon index of Γ =1.82^{+0.29}_{-0.27} and 1.94^{+0.31}_{-0.29}, respectively, which is consistent with the mean value of ∼1.9 found for quasars at all redshifts. The rest-frame 2-10 keV luminosity of VDES J0224-4711 is L_{2-10 keV} = (2.92± 0.43)x 10^{45} erg s^{-1}, which makes this quasar one of the most X-ray luminous quasars at z > 5.5 and the most X-ray luminous quasar at z > 6.5, with a luminosity 6 times and 2.5 times larger than ULAS J1120+0641 (z = 7.08) and ULAS J1342+0928 (z = 7.54), respectively. The X-ray-to-optical power-law slopes of the nine quasars are consistent with the previously observed anticorrelation of αox with UV luminosity L_{2500 Å}. We find no evidence for evolution of αox with redshift when the anticorrelation with UV luminosity is taken into account. Similar to previous studies at z ∼ 6, we have found remarkably consistent X-ray spectral properties between low-redshift quasars (z ∼ 1) and high-redshift quasars. Our results add further evidence to the picture that the observable properties of high-luminosity quasars over the UV-to-X-ray spectral region have not evolved significantly from z ∼ 7 to the present day and that quasars comparable to local versions existed 800 Myr after the big bang.