XMM-Newton observations of the extremely X-ray luminous quasar CFHQS
  J142952+544717=SRGE J142952.1+544716 at redshift z=6.18
  We present results from a 20 ks XMM-Newton DDT observation of the radio-load
quasar CFHQS J142952+544717 at z=6.18, whose extreme X-ray luminosity was
recently revealed by the SRG/eROSITA telescope in the course of its first
all-sky survey. The quasar has been confidently detected with a total of $\sim
1400$ net counts in the 0.2-10 keV energy band (1.4 to 72 keV in the object's
rest frame). Its measured spectrum is unusually soft and can be described by an
absorbed power-law model with a photon index of $\Gamma = 2.5\pm0.2$. There are
no signs of a high-energy cutoff or reflected component, with an 90 % upper
limit on the fluorescence iron K$\alpha$ equivalent width of $\approx 290$ eV
and the corresponding upper limit on the iron K-edge absorption depth of 0.6.
We have detected, at the $> 95\%$ confidence level, an excess absorption above
the Galactic value, corresponding to a column density $N_H= 3\pm2 \times
10^{22}$ cm$^{-2}$ of material located at z=6.18. The intrinsic luminosity of
CFHQS J142952+544717 in the 1.4 to 72 keV energy band is found to be
$5.5_{-0.6}^{+0.8} \times 10^{46}$ erg s$^{-1}$. We did not detect
statistically significant flux changes between two SRG scans and the XMM-Newton
observation, spanning over $\sim 7.5$ months, implying that the quasar remained
at this extremely high luminosity level for at least a month in its rest frame.
We put forward the hypothesis that the extreme X-ray properties of CFHQS
J142952+544717 are associated with inverse Compton scattering of cosmic
microwave background photons (at z=6.18) in its relativistic jets.
