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 $\times 10^{26}$ W Hz$^{-1}$ at 5 GHz) compared to the optical one (1.8 $\times 10^{24}$ W Hz$^{-1}$ at 4400 A) makes it the most distant radio-loud quasar observed so far, with a radio loudness R$\sim$70 (R$=L_{5GHz}/L_{4400A}$). Moreover, the large bolometric luminosity associated to the accretion disk (L$_{bol}$=7.5 $\times 10^{46}$ erg s$^{-1}$) suggests the presence of a supermassive black hole with a large mass ($\sim$10$^9$ M$_\odot$) when the Universe was less than a billion years old. Combining the new radio observation from RACS with archival radio data at the same frequency, we found that the flux density of the source may have varied by a factor of $\sim$2, which could suggest the presence of a relativistic jet oriented towards the line of sight, i.e. 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.