Ighina et al. present radio and X-ray observations of the recently discovered high-redshift (z=6.13) radio-powerful quasar RACS J032021.44-352104.1, using the upgraded Giant Metrewave Radio Telescope, the Australia Telescope Compact Array, the Long Baseline Array, and Chandra. The observed radio properties are in line with what is typically observed in high-redshift radio quasars. Despite the relatively low X-ray flux observed, the intrinsic luminosity in the 2–10 keV rest frame is markedly high, making RACS J032021.44-352104.1 one of the most luminous quasars currently known at z > 5.5. By analyzing the overall spectral energy distribution of the quasar, the team found that the remarkably soft X-ray emission (1) cannot be produced by relativistic jets, even when relativistic boosting is considered, and (2) is consistent with expectations for a super-Eddington accreting supermassive black hole. The Eddington limit is the theoretical maximum rate at an object can accrete matter — reached when outward radiation pressure equals the infall under gravity. In the case of Super-Eddington accretion, which can occur with non-spherical symmetry, the excess material is expelled through powerful outflows, such as jets and winds. If such a high accretion rate was confirmed, this source would be a unique laboratory to study high accretion in the early Universe and could help resolve some challenges inherent in early black hole growth paradigms.

VLBI observations of RACS J0320−35 were performed with the Australian the Long Baseline Array in October 2022 at 2.3 GHz and 8.4 GHz. At 2.3 GHz, a 5σ radio signal is present ∼ 16mas from the optical position of the quasar (the white cross in the figure above). Even though the signal-to-noise ratio of this radio emission alone is relatively low, the close position to the expected core is an indication that this emission is true and likely produced by the inner-most regions of the jets, close to the accretion disc. At 8.4 GHz, there is no significant radio signal at the target position, nor within ∼200 mas radius around it allowing constraints to be placed on the spectral index of the core. The image shows LBA images of RACS J0320−35 at 2.3 (left) and 8.4 GHz (right). The white cross shows the optical position of RACS J0320−35.