Discovery of an X-Ray Luminous Radio-loud Quasar at z = 3.4: A Possible Transitional Super-Eddington Phase

Abstract We report the multiwavelength properties of eROSITA Final Equatorial Depth Survey (eFEDS) J084222.9+001000 (hereafter ID830), a quasar at z = 3.4351, identified as the most X-ray luminous radio-loud quasar in the eFEDS field. ID830 shows a rest-frame 0.5–2 keV luminosity of log ( L 0.5 − 2 keV / erg s − 1 ) = 46.20 ± 0.12 , with a steep X-ray photon index (Γ = 2.43 ± 0.21), and a significant radio counterpart detected with the Very Large Array FIRST 1.4 GHz and Very Large Array Sky Survey 3 GHz bands. The rest-frame UV to optical spectra from Sloan Digital Sky Survey and Subaru/MOIRCS J band show a dust-reddened quasar feature with A V = 0.39 ± 0.08 mag, and the expected bolometric active galactic nuclei luminosity from the dust-extinction-corrected UV luminosity reaches L bol,3000Å = (7.62 ± 0.31) × 10 46 erg s −1 . We estimate a black hole mass of M BH = (4.40 ± 0.72) × 10 8 M ⊙ based on the Mg II λ 2800 emission-line width, and Eddington ratios from the dust-extinction-corrected UV continuum luminosity and X-ray luminosity that reach λ Edd,UV = 1.44 ± 0.24 and λ Edd,X = 12.8 ± 3.9, respectively, both indicating super-Eddington accretion. ID830 shows a high ratio of UV to X-ray luminosities, α OX = −1.20 ± 0.07 (or α OX = −1.42 ± 0.07 after correcting for jet-linked X-ray excess), higher than quasars and little red dots in the super-Eddington phase with similar UV luminosities, with α OX < −1.8. Such a high α OX suggests the coexistence of a prominent radio jet and X-ray corona in this high-Eddington-accretion phase. We propose that ID830 may be in a transitional phase after an accretion burst, evolving from a super-Eddington to a sub-Eddington state, which could naturally describe the high α OX .