X-ray properties of z>6.5 quasars
  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 ($>$4$\sigma$) significance level: VDES J0224$-$4711(z=6.53) at $9\sigma$
and PSO J159$-$02 ($z=6.38$) at $8\sigma$. They have a photon index of
$\Gamma=1.82^{+0.29}_{-0.27}$ and $\Gamma=1.94^{+0.31}_{-0.29}$ 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
$L_{2-10\mathrm{keV}} = (2.92\pm0.43)\times10^{45}\;\mathrm{erg\;s^{-1}}$,
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 anti-correlation of $\alpha_{ox}$ with
UV luminosity $L_{2500\mathrm{\AA}}$. We find no evidence for evolution of
$\alpha_{ox}$ 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 $z\sim7$ to the present day and that
quasars comparable to local versions existed 800 million years after the big
bang.
