Discovery of the Most X-ray Luminous Quasar SRGE J170245.3+130104 at Redshift $$\boldsymbol{z\approx 5.5}$$

During the first all-sky survey of the SRG orbital observatory, the X-ray source SRGE J170245.3+130104 was discovered with the eROSITA telescope on March 13–15, 2020. Its optical counterpart was identified by photometric attributes as a candidate for distant quasars at $$z\approx 5.5$$ . The spectroscopic observations of the object carried out in August and September 2020 at the 6-m BTA telescope with the SCORPIO-II instrument confirmed that SRGE J170245.3+130104 is a quasar at redshift $$z_{\textrm{{spec}}}=5.466\pm 0.003$$ . According to the eROSITA data obtained during the first sky survey, the X-ray luminosity of the quasar is $$3.6^{+2.1}_{-1.5}\times 10^{46}$$ erg s $${}^{-1}$$ in the 2–10 keV energy band, while its X-ray spectrum can be approximately fitted by a power law with a slope $$\Gamma=1.8^{+0.9}_{-0.8}$$ . The quasar was detected again with the eROSITA telescope half a year later (on September 13–14, 2020) during the second sky survey, with its X-ray luminosity, probably, having decreased approximately by a factor of 2 at a confidence level $${\approx}1.9\sigma$$ . SRGE J170245.3+130104 has turned out to be the most X-ray luminous quasar among all of the known quasars at redshifts $$z>5$$ . At the same time, it is also one of the radio loudest distant quasars (radio loudness $$R\sim 10^{3}$$ ), which may imply that this is a blazar. We present a catalog of all the quasars at $$z>5$$ detected in X-rays to date.