2020A&A...641A..85S

We present Giant Metrewave Radio Telescope (GMRT) 323MHz radio continuum observations toward 13 radio-loud quasars at z>5, sampling the low-frequency synchrotron emission from these objects. Among the 12 targets successfully observed, we detected 10 above 4σ significance, while 2 remain undetected. All of the detected sources appear as point sources. Combined with previous radio continuum detections from the literature, 9 quasars have power-law spectral energy distributions throughout the radio range; for some the flux density drops with increasing frequency while it increases for others. Two of these sources appear to have spectral turnover. For the power-law-like sources, the power-law indices have a positive range between 0.18 and 0.67 and a negative values between -0.90 and -0.27. For the turnover sources, the radio peaks around ∼1 and ∼10GHz in the rest frame, the optically thin indices are -0.58 and -0.90, and the optically thick indices are 0.50 and 1.20. A magnetic field and spectral age analysis of SDSS J114657.59+403708.6 at z=5.01 may indicate that the turnover is not caused by synchrotron self-absorption, but rather by free-free absorption by the high-density medium in the nuclear region. Alternatively, the apparent turnover may be an artifact of source variability. Finally, we calculated the radio loudness R_2500{A} for our sample, which spans a very wide range from 12_–13⁺¹³ to 4982_–254⁺²⁷⁹.