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

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 23h 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 (αr = 1.24, with Sν ∝ ν−αr), 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.

High-resolution Imaging of Two Radio Quasars at the End of the Reionization Epoch

“…This constrains the spectral index of the pc-scale feature to 𝛼 < −1.2, which is consistent with the value found for the kpc-scale radio emission [8]. Detailed analysis [8] showed that the radio emission of J2318−3113 likely originates from a recently formed quasar jet, as fitting both a curved (peaked) and a double power law model to the radio spectrum implies a young kinetic (∼ 500 yr) and radiative age (≲ 9 × 10 4 yr).…”

“…The source VIKING J231818.35−311346.3 (J2318−3113) was identified as a quasar at redshift 𝑧 = 6.44 [1], and detected in the radio domain at 888 MHz in the Galaxy and Mass Assembly and Rapid ASKAP Continuum Surveys [7]. J2318−3113 was found to be a radio-loud (radio loudness parameter 𝑅 = 70, [7]) steep spectrum quasar, with a spectral index of 𝛼 = −1.24 between 888 MHz and 5.5 GHz ( [8], following the 𝑆 ∼ 𝜈 𝛼 convention, where 𝑆 is the flux density and 𝜈 is the frequency).…”

High-resolution Imaging of Two Radio Quasars at the End of the Reionization Epoch

“…5 image noise) was detected at 4.7 GHz [19]. This constrains the spectral index of the pc-scale feature to < −1.2, which is consistent with the value found for the kpc-scale radio emission [8]. These findings imply that the radio emission originates from a recently formed young quasar jet in the early Universe [19].…”

“…The source VIKING J231818.35−311346.3 (J2318−3113) was identified as a quasar at redshift = 6.44 [1], and detected in the radio domain at 888 MHz in the Galaxy and Mass Assembly and Rapid ASKAP Continuum Surveys [7]. J2318−3113 was found to be a radio-loud (radio loudness parameter = 70, [7]) steep spectrum quasar, with a spectral index of = −1.24 between 888 MHz and 5.5 GHz ( [8], following the ∼ convention, where is the flux density and is the frequency).…”

The GLEAMing of the first supermassive black holes: II. A new sample of high-redshift radio galaxy candidates

“…The search for ultra-high-redshift radio-loud AGN has now progressed beyond z = 6. Radio-loud quasars have recently been identified near the end of the EoR (z ∼ 6.5): VIK J2318−3113 at z = 6.44 (Ighina et al 2021(Ighina et al , 2022) and P172+18 at z = 6.82 (Bañados et al 2021;Momjian et al 2021). In addition, Belladitta et al (2020) discovered PSO J0309+27 at z = 6.10; this radioloud source is the most distant known blazar.…”