Discovery of the Most X-ray Luminous Quasar SRGE J170245.3+130104 at Redshift \boldsymbol{z≈ 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 $z≈ 5.5$. 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 $z_{\textrm{{spec}}}=5.466± 0.003$. According to the eROSITA data obtained during the first sky survey, the X-ray luminosity of the quasar is $3.6^{+2.1}_{-1.5}× 10^{46}$ 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 $Γ=1.8^{+0.9}_{-0.8}$. 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 ${≈}1.9σ$. 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 $R∼ 10³$), 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.