2021A&A...647L..11I

We report the 888 MHz radio detection in the Rapid ASKAP Continuum Survey (RACS) of VIK J2318-3113, a z=6.44 quasar. Its radio luminosity (1.2x10²⁶W/Hz at 5GHz) compared to the optical luminosity (1.8x10²⁴W/Hz at 4400Å) makes it the most distant radio-loud quasar observed so far, with a radio loudness R∼70 (R=L_5GHz/L_4400Å). Moreover, the high bolometric luminosity of the source (L_bol=7.4x10⁴⁶erg/s) suggests the presence of a supermassive black hole with a high mass (≥6x10⁸M_☉) at a time when the Universe was younger than a billion years. Combining the new radio data from RACS with previous ASKAP observations at the same frequency, we found that the flux density of the source may have varied by a factor of ∼2, which could suggest the presence of a relativistic jet oriented towards the line of sight, that is, a blazar nature. However, currently available radio data do not allow us to firmly characterise the orientation of the source. Further radio and X-ray observations are needed.