Discovery of the most X-ray luminous quasar SRGE J170245.3+130104 at redshift z$\approx5.5$

TL;DR

This paper reports the discovery and detailed X-ray analysis of SRGE J170245.3+130104, the most X-ray luminous quasar at redshift 5.5, highlighting its potential as a blazar and its significance among high-redshift quasars.

Contribution

It presents the first detection and characterization of the most X-ray luminous quasar at z>5, including its luminosity, spectrum, and variability, expanding knowledge of early universe quasars.

Findings

Most X-ray luminous quasar at z>5

Detected X-ray luminosity of approximately 3.6 x 10^46 erg/s

Possible classification as a blazar due to high radio-loudness

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 $z\approx5.5$. 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.