Comprehensive view on a z~6.5 radio-loud QSO: from the radio to the optical/NIR to the X-ray band

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 (RL) quasi stellar object (QSO), one of the most distant currently known in this class. The work is based on newly obtained (uGMRT, ATCA, Chandra) as well as archival (GNIRS and X-Shooter) dedicated observations that have not been published yet. 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 νₚₑₐₖ ~ 650 MHz which is unlikely to be associated with self-synchrotron absorption. From the NIR spectrum we derived the mass of the central black hole, MBH=8.1+6.8-5.6 × 10⁸ M_, and the Eddington ratio, λEDD = 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 ηd~0.1. However, by assuming ηd~0.3, as expected for jetted systems, the mass observed would challenge current theoretical models of black hole formation.