Radio detection of VIK J2318-3113, the most distant radio-loud quasar (z = 6.44)

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 (1.2 × 1026 W Hz-1 at 5 GHz) compared to the optical luminosity (1.8 × 1024 W Hz-1 at 4400 A) makes it the most distant radio-loud quasar observed so far, with a radio loudness R ∼ 70 (R = L5 GHz/L4400A). Moreover, the high bolometric luminosity of the source (Lbol = 7.4 × 1046 erg s-1) suggests the presence of a supermassive black hole with a high mass (6 × 108 Mpdbl) 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.