2024A&A...687A.242I

We present a multi-wavelength analysis, from the radio to the X-ray band, of the redshift z = 6.44 VIK J2318-31 radio-loud quasi-stellar object, one of the most distant currently known of this class. The work is based on newly obtained observations (uGMRT, ATCA, and Chandra) as well as dedicated archival observations that have not yet been published (GNIRS and X-shooter). Based on the observed X-ray and radio emission, its relativistic jets are likely young and misaligned from our line of sight. Moreover, we can confirm, with simultaneous observations, the presence of a turnover in the radio spectrum at ν_peak ∼ 650 MHz that is unlikely to be associated with self-synchrotron absorption. From the near-infrared spectrum we derived the mass of the central black hole, $ M_{\mathrm{BH}}=8.1^{+6.8}_{-5.6} \times 10^8\,{M_{\odot}} $, and the Eddington ratio, $ \lambda_{\mathrm{EDD}} = 0.8^{+0.8}_{-0.6} $, using broad emission lines as well as an accretion disc model fit to the continuum emission. Given the high accretion rate, the presence of a ∼8 × 10⁸ M_⊙ black hole at z = 6.44 can be explained by a seed black hole (∼10⁴ M_⊙) that formed at z ∼ 25, assuming a radiative efficiency η_d ∼ 0.1. However, by assuming η_d ∼ 0.3, as expected for jetted systems, the mass observed would challenge current theoretical models of black hole formation.