Kavli Affiliate: R. G. McMahon
| Summary:
We present XMM-Newton X-ray observations and analysis of three DES $z>6.5$ 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 ($>$4$σ$) significance level: VDES J0224$-$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 $Γ=1.94^+0.31_-0.29$ respectively, which is consistent with the mean value of $sim1.9$ found for quasars at all redshifts. The rest-frame $2-10$ keV luminosity of VDES J0224$-$4711, is $L_2-10mathrmkeV = (2.92pm0.43)times10^45;mathrmerg;s^-1$, 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 anti-correlation of $α_ox$ with UV luminosity $L_2500mathrmÅ$. We find no evidence for evolution of $α_ox$ with redshift when the anti-correlation with UV luminosity is taken into account. Similar to previous studies at z$sim$6 we have found remarkably consistent X-ray spectral properties between low (z$sim$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 $zsim7$ to the present day and that quasars comparable to local versions existed 800 million years after the big bang.
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