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

Abstract

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 × 10^26 W Hz^−1 at 5 GHz) compared to the optical luminosity (1.8 × 10^24 W Hz^−1 at 4400 Å) makes it the most distant radio-loud quasar observed so far, with a radio loudness R ∼ 70 (R = L_5 GHz/L4400 Å). Moreover, the high bolometric luminosity of the source (L_bol = 7.4 × 10^46 erg s^−1) suggests the presence of a supermassive black hole with a high mass (≳6 × 10^8 M⊙) 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.