Discovery of the most X-ray luminous quasar SRGE J170245.3+130104 at redshift z≈5.5
Abstract
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≈5.5. 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× 1046 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 =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 ≈ 1.9σ 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 R103), 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.