Letter to the Editor

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

¹ INAF – Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milan, Italy
² DiSAT – Università degli Studi dell’ Insubria, Via Valleggio 11, 22100 Como, Italy
e-mail: lighina@uninsubria.it
³ International Centre for Radio Astronomy Research, Curtin University, 1 Turner Avenue, Bentley, WA 6102, Australia

Received: 15 January 2021
Accepted: 1 March 2021

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 × 10²⁶ W Hz⁻¹ at 5 GHz) compared to the optical luminosity (1.8 × 10²⁴ 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/L_4400 Å). Moreover, the high bolometric luminosity of the source (L_bol = 7.4 × 10⁴⁶ erg s⁻¹) suggests the presence of a supermassive black hole with a high mass (≳6 × 10⁸ 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.