L. Ighina, A. Caccianiga, T. Connor, A. Moretti, F. Pacucci, C. Reynolds, J. Afonso, B. Arsioli, S. Belladitta, J. W. Broderick, D. Dallacasa, R. Della Ceca, F. Haardt, E. Lambrides, J. K. Leung, A. Lupi, I. Matute, F. Rigamonti, P. Severgnini, N. Seymour, F. Tavecchio, C. Vignali

Abstract
We present radio and X-ray observations of the recently discovered z = 6.13 radio-powerful quasar RACS J032021.44‑352104.1 using the upgraded Giant Metrewave Radio Telescope, the Australia Telescope Compact Array, the Australian Large Baseline Array, and Chandra. The observed radio properties are in line with what is typically observed in high-z radio quasars (α_r = 0.72 ± 0.02 and L_{1.4 GHz} = 5.8 ± 0.9 × 10²⁶ W Hz^‑1). Despite the relatively low X-ray flux observed, F_{0.5‑7.0 keV} = 2.3 ± 0.5 × 10^‑14 erg s^‑1 cm^‑2, the intrinsic luminosity in the 2–10 keV rest frame is markedly high, L2-10keV=1.8‑0.7+1.1×1046 erg s^‑1, making RACS J032021.44‑352104.1 one of the most luminous quasars currently known at z > 5.5. The high X-ray luminosity is largely driven by an extrapolation to energies below the observable X-ray window with Chandra and the slope derived in the 0.5–7 keV band (or 3.5–50 keV in the rest frame; Γ_X = 3.3 ± 0.4). By analyzing the overall spectral energy distribution of the quasar, we 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. 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.

Keywords
galaxies active / relativity / gas / astronomy x rays / astronomy radio / galaxies quasars

The Astrophysical Journal
Volume 990, Number L56, Page 13
2025 September

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