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Register a new userDiscovery of the most X-ray luminous quasar SRGE J170245.3+130104 at redshift z$approx5.5$
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Added by
George Khorunzhev Dr.
Publication date
2021
fields
Physics
and research's language is
English
Authors
G.A. Khorunzhev
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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 $zapprox5.5$. 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 $approx 1.9sigma$ 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 $Rsim10^3$), 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.
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We present results from a 20 ks XMM-Newton DDT observation of the radio-load quasar CFHQS J142952+544717 at z=6.18, whose extreme X-ray luminosity was recently revealed by the SRG/eROSITA telescope in the course of its first all-sky survey. The quasar has been confidently detected with a total of $sim 1400$ net counts in the 0.2-10 keV energy band (1.4 to 72 keV in the objects rest frame). Its measured spectrum is unusually soft and can be described by an absorbed power-law model with a photon index of $Gamma = 2.5pm0.2$. There are no signs of a high-energy cutoff or reflected component, with an 90 % upper limit on the fluorescence iron K$alpha$ equivalent width of $approx 290$ eV and the corresponding upper limit on the iron K-edge absorption depth of 0.6. We have detected, at the $> 95%$ confidence level, an excess absorption above the Galactic value, corresponding to a column density $N_H= 3pm2 times 10^{22}$ cm$^{-2}$ of material located at z=6.18. The intrinsic luminosity of CFHQS J142952+544717 in the 1.4 to 72 keV energy band is found to be $5.5_{-0.6}^{+0.8} times 10^{46}$ erg s$^{-1}$. We did not detect statistically significant flux changes between two SRG scans and the XMM-Newton observation, spanning over $sim 7.5$ months, implying that the quasar remained at this extremely high luminosity level for at least a month in its rest frame. We put forward the hypothesis that the extreme X-ray properties of CFHQS J142952+544717 are associated with inverse Compton scattering of cosmic microwave background photons (at z=6.18) in its relativistic jets.
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