Constraining the radio properties of the z = 6.44 QSO VIK J2318−3113

¹ INAF, Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy
e-mail: lighina@uninsubria.it
² DiSAT, Università degli Studi dell’Insubria, Via Valleggio 11, 22100 Como, Italy
³ Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
⁴ CSIRO Space and Astronomy, PO Box 76 Epping, NSW 1710, Australia
⁵ ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), Hawthorn, VIC 3122, Australia
⁶ International Centre for Radio Astronomy Research, Curtin University, 1 Turner Avenue, Bentley, WA 6102, Australia
⁷ ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), The University of Sydney, NSW 2006, Australia
⁸ Shanghai Astronomical Observatory, Chinese Academy of Sciences, Nandan Road 80, Shanghai 200030, PR China
⁹ Key Laboratory of Cognitive Radio and Information Processing, Guilin University of Electronic Technology, 541004 Guilin, PR China
¹⁰ CSIRO Space and Astronomy, PO Box 1130 Bentley, WA 6102, Australia
¹¹ School of Natural Sciences, University of Tasmania, Private Bag 37, Hobart, TAS 7001, Australia
¹² ARC Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
¹³ Department of Physics and Electronics, Rhodes University, PO Box 94 Makhanda 6140, South Africa

Received: 23 November 2021
Accepted: 11 March 2022

Abstract

The recent detection of the quasi-stellar object (QSO) VIKING J231818.3−311346 (hereafter VIK J2318−3113) at redshift z = 6.44 in the Rapid ASKAP Continuum Survey (RACS) uncovered its radio-loud nature, making it one of the most distant known to date in this class. By using data from several radio surveys of the Galaxy And Mass Assembly 23^h field and from a dedicated follow-up, we were able to constrain the radio spectrum of VIK J2318−3113 in the observed range ∼0.1–10 GHz. At high frequencies (0.888–5.5 GHz in the observed frame) the QSO presents a steep spectrum (α_r = 1.24, with S_ν ∝ ν^−α_r), while at lower frequencies (0.4–0.888 GHz in the observed frame) it is nearly flat. The overall spectrum can be modelled by either a curved function with a rest-frame turnover around 5 GHz, or with a smoothly varying double power law that is flat below a rest-frame break frequency of about 20 GHz and above which it significantly steepens. Based on the model adopted, we estimated that the radio jets of VIK J2318−3113 must be a few hundred years old in the case of a turnover, or less than a few × 10⁴ years in the case of a break in the spectrum. Having multiple observations at two frequencies (888 MHz and 5.5 GHz), we further investigated the radio variability previously reported for this source. We found that the marginally significant flux density variations are consistent with the expectations from refractive interstellar scintillation, even though relativistic effects related to the orientation of the source may still play a non-negligible role. Further radio and X-ray observations are required to conclusively discern the nature of this variation.