Chandra Detection of Three X-ray Bright Quasars at z>5
  We report Chandra detection of three UV bright radio quiet quasars at
$z\gtrsim5$. We have collected a sufficient number of photons to extract an
X-ray spectrum of each quasar to measure their basic X-ray properties, such as
the X-ray flux, power law photon index ($\Gamma$), and optical-to-X-ray
spectral slope ($\alpha_{\rm OX}$). J074749+115352 at $z=5.26$ is the X-ray
brightest radio-quiet quasar at $z>5$. It may have a short timescale variation
(on a timescale of $\sim3800\rm~s$ in the observer's frame, or $\sim600\rm~s$
in the rest frame) which is however largely embedded in the statistical noise.
We extract phase folded spectra of this quasar. There are two distinguishable
states: a "high soft" state with an average X-ray flux $\sim2.7$ times of the
"low hard" state, and a significantly steeper X-ray spectral slope
($\Gamma=2.40_{-0.32}^{+0.33}$ vs $1.78_{-0.24}^{+0.25}$). We also compare the
three quasars detected in this paper to other quasar samples. We find that
J074749+115352, with a SMBH mass of $M_{\rm
SMBH}\approx1.8\times10^9\rm~M_\odot$ and an Eddington ratio of $\lambda_{\rm
Edd}\approx2.3$, is extraordinarily X-ray bright. It has an average
$\alpha_{\rm OX}=-1.46\pm0.02$ and a 2-10 keV bolometric correction factor of
$L_{\rm bol}/L_{\rm2-10keV}=42.4\pm5.8$, both significantly depart from some
well defined scaling relations. We compare $\Gamma$ of the three quasars to
other samples at different redshifts, and do not find any significant redshift
evolution based on the limited sample of $z>5$ quasars with reliable
measurements of the X-ray spectral properties.
