Discovery of the most X-ray luminous quasar SRGE J170245.3+130104 at
  redshift z$\approx5.5$
  SRGE J170245.3+130104 was discovered by the eROSITA telescope aboard the SRG
space observatory on March 13-15, 2020 during the first half-year scan of its
all-sky X-ray survey. The optical counterpart of the X-ray source was
photometrically identified as a distant quasar candidate at $z\approx5.5$.
Follow-up spectroscopic observations, done in August/September 2020 with the
SCORPIO-II instrument at the BTA 6-m telescope, confirmed that SRGE
J170245.3+130104 is a distant quasar at redshift z=5.466. The X-ray luminosity
of the quasar during the first half-year scan of the eROSITA all-sky survey was
$3.6^{+2.1}_{-1.5}\times 10^{46}$ erg/s (in the 2-10 keV energy range), whereas
its X-ray spectrum could be described by a power law with a slope of
$\Gamma=1.8^{+0.9}_{-0.8}$. Six months later (September 13-14, 2020), during
the second half-year scan of the eROSITA all-sky survey, the quasar was
detected again and its X-ray luminosity had decreased by a factor of 2 (at the
$\approx 1.9\sigma$ confidence level). The SRGE J170245.3+130104 proves to be
the most X-ray luminous among all known X-ray quasars at $z>5$. It is also one
of the radio-loudest distant quasars (with radio-loudness $R\sim10^3$), which
may imply that it is a blazar. In the Appendix, we present the list of all
$z>5$ quasars detected in X-rays to date.
