2023A&A...678A.128G

We present the low-frequency radio spectra of nine high-redshift quasars at 5.6 ≤ z ≤ 6.6 using the Giant Metre Radio Telescope band-3, -4, and -5 observations (∼300-1200 MHz), archival Low Frequency Array (LOFAR; 144 MHz), and Very Large Array (VLA; 1.4 and 3 GHz) data. Five of the quasars in our sample have been discovered recently, representing some of the highest redshift radio bright quasars known at low frequencies. We model their radio spectra to study their radio emission mechanism and age of the radio jets by constraining the spectral turnover caused by synchrotron self-absorption (SSA) or free-free absorption (FFA). Besides J0309+2717, a blazar at z = 6.1, our quasars show no sign of a spectral flattening between 144 MHz and a few gigahertz, indicating there is no strong SSA or FFA absorption in the observed frequency range. However, we find a wide range of spectral indices between -1.6 and 0.05, including the discovery of 3 potential ultra-steep spectrum quasars. Using further archival Very Long Baseline Array data, we confirm that the radio spectral energy distribution of the blazar J0309+2717 likely turns over at a rest-frame frequency of 0.6-2.3 GHz (90-330 MHz observed frame), with a high-frequency break indicative of radiative ageing of the electron population in the radio lobes. Ultra-low frequency data below 50 MHz are necessary to constrain the absorption mechanism for J0309+2717 and the turnover frequencies for the other high-z quasars in our sample. A relation between linear radio jet size and turnover frequency has been established at low redshifts. If this relation were to hold at high redshifts, the limits on the turnover frequency of our sample suggest the radio jet sizes must be more extended than the typical sizes observed in other radio-bright quasars at similar redshift. To confirm this, deep radio follow-up observations with high spatial resolution are required.