SRG/eROSITA uncovers the most X-ray luminous quasar at z>6

Abstract

We report the discovery of X-ray emission from CFHQS J142952+544717, the most distant known radio-loud quasar at z=6.18, on Dec. 10--11, 2019 with the eROSITA telescope on board the SRG satellite during its ongoing all-sky survey. The object was identified by cross-matching an intermediate SRG/eROSITA source catalog with the Pan-STARRS1 distant quasar sample at 5.6 < z < 6.7. The measured flux $\sim 8\times 10^{-14}$ erg cm${}^{-2}$ s${}^{-1}$ in the 0.3--2 keV energy band corresponds to an X-ray luminosity of $2.6_{-1.0}^{+1.7}\times 10^{46}$ erg s${}^{-1}$ in the 2--10 keV rest-frame energy band, which renders CFHQS J142952+544717 the most X-ray luminous quasar ever observed at z > 6. Combining our X-ray measurements with archival and new photometric measurements in other wavebands (radio to optical), we estimate the bolometric luminosity of this quasar at $\sim (2$--$3)\times 10^{47}$ erg s${}^{-1}$. Assuming Eddington limited accretion and isotropic emission, we infer a lower limit on the mass of the supermassive black hole of $\sim 2\times 10^9{M}_{\odot }$. The most salient feature of CFHQS J142952+544717 is its X-ray brightness relative to the optical/UV emission. We argue that it may be linked to its radio-loudness (although the object is not a blazar according to its radio properties), specifically to a contribution of inverse Compton scattering of cosmic microwave background photons off relativistic electrons in the jets. If so, CFHQS J142952+544717 might be the tip of the iceberg of high-z quasars with enhanced X-ray emission, and SRG/eROSITA may find many more such objects during its 4 year all-sky survey.

9 pages, 4 figures, 1 table. Accepted for publication in MNRAS

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