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We report the discovery of X-ray emission from CFHQS J142952+544717, the mostdistant known radio-loud quasar at z=6.18, on Dec. 10--11, 2019 with theeROSITA telescope on board the SRG satellite during its ongoing all-sky survey.The object was identified by cross-matching an intermediate SRG/eROSITA sourcecatalog with the Pan-STARRS1 distant quasar sample at 5.6 < z < 6.7. Themeasured flux $\sim 8 \times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$ in the 0.3--2 keVenergy band corresponds to an X-ray luminosity of $2.6^{+1.7}_{-1.0}\times10^{46}$ erg s$^{-1}$ in the 2--10 keV rest-frame energy band, which rendersCFHQS J142952+544717 the most X-ray luminous quasar ever observed at z > 6.Combining our X-ray measurements with archival and new photometric measurementsin other wavebands (radio to optical), we estimate the bolometric luminosity ofthis quasar at $\sim (2$--$3) \times 10^{47}$ erg s$^{-1}$. Assuming Eddingtonlimited accretion and isotropic emission, we infer a lower limit on the mass ofthe supermassive black hole of $\sim 2\times 10^9 M_\odot$. The most salientfeature of CFHQS J142952+544717 is its X-ray brightness relative to theoptical/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 cosmicmicrowave background photons off relativistic electrons in the jets. If so,CFHQS J142952+544717 might be the tip of the iceberg of high-z quasars withenhanced X-ray emission, and SRG/eROSITA may find many more such objects duringits 4 year all-sky survey.

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