Discovery of an X-ray Luminous Radio-Loud Quasar at $z=3.4$: A Possible Transitional Super-Eddington Phase

Sakiko Obuchi, Kohei Ichikawa, Satoshi Yamada, Nozomu Kawakatu, Teng Liu, Naoki Matsumoto, Andrea Merloni, Kosuke Takahashi, Ingyin Zaw, Xiaoyang Chen, Kazuhiro Hada, Zsofi Igo, Hyewon Suh, Julien Wolf

First listed 2025-11-07 | Last updated 2025-11-07

Abstract

We report the multiwavelength properties of eFEDS J084222.9+001000 (hereafter ID830), a quasar at $z=3.4351$, identified as the most X-ray luminous radio-loud quasar in the eROSITA Final Equatorial Depth Survey (eFEDS) field. ID830 shows a rest-frame 0.5-2 keV luminosity of $\log (L_\mathrm{0.5-2\,keV}/\mathrm{erg}~\mathrm{s}^{-1}) = 46.20 \pm 0.12$, with a steep X-ray photon index ($Γ=2.43 \pm 0.21$), and a significant radio counterpart detected with VLA/FIRST 1.4 GHz and VLASS 3 GHz bands. The rest-frame UV to optical spectra from SDSS and Subaru/MOIRCS $J$-band show a dust reddened quasar feature with $A_\mathrm{V} = 0.39 \pm 0.08$ mag and the expected bolometric AGN luminosity from the dust-extinction-corrected UV luminosity reaches $L_\mathrm{bol,3000}= (7.62 \pm 0.31) \times 10^{46}$ erg s$^{-1}$. We estimate the black hole mass of $M_\mathrm{BH} = (4.40 \pm 0.72) \times 10^{8} M_{\odot}$ based on the MgII$λ$2800 emission line width, and an Eddington ratio from the dust-extinction-corrected UV continuum luminosity reaches $λ_\mathrm{Edd,UV}=1.44 \pm 0.24$ and $λ_{\mathrm{Edd,X}} = 12.8 \pm 3.9$ from the X-ray luminosity, both indicating the super-Eddington accretion. ID830 shows a high ratio of UV-to-X-ray luminosities, $α_\mathrm{OX}=-1.20 \pm 0.07$ (or $α_\mathrm{OX}=-1.42 \pm 0.07$ after correcting for jet-linked X-ray excess), higher than quasars and little red dots in super-Eddington phase with similar UV luminosities, with $α_\mathrm{OX}<-1.8$. Such a high $α_\mathrm{OX}$ 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 $α_\mathrm{OX}$.

Short digest

Multiwavelength follow-up identifies eFEDS J084222.9+001000 (ID830) at z=3.4351 as the most X-ray luminous radio-loud quasar in the eFEDS field. It shows log L0.5–2keV=46.20±0.12 with a steep Γ=2.43±0.21, radio detections (FIRST 1.4 GHz; VLASS 3 GHz), modest reddening AV=0.39±0.08, Lbol,3000=(7.62±0.31)×10^46 erg s−1, and MBH=(4.40±0.72)×10^8 M⊙ from Mg II. The inferred Eddington ratios are λEdd,UV=1.44±0.24 and λEdd,X=12.8±3.9, and the source is unusually X-ray bright for its UV luminosity with αOX=−1.20 (−1.42 after jet-linked correction), unlike super-Eddington quasars and little red dots that typically have αOX<−1.8. The authors argue ID830 is a transitional post-burst phase where a prominent jet and vigorous X-ray corona coexist as the system evolves from super- to sub-Eddington accretion.

Key figures to inspect

• Figure 1: Locate ID830 on the L0.5–2 keV–redshift plane to see it uniquely occupies the extreme luminosity region targeted by the selection, underscoring its rarity within the eFEDS-WERGS sample.
• Figure 2: Inspect the eROSITA X-ray spectrum and best-fit components to verify the steep photon index (Γ≈2.4) and the lack of heavy absorption, key to the X-ray-bright, soft-state interpretation.
• Figure 3: Compare SDSS+MOIRCS spectra to the reddened quasar template to read off AV≈0.39 and how the extinction correction sets Lbol,3000 used for λEdd,UV and for the αOX evaluation.
• Figure 4: Use the PyQSOfit line decompositions—especially Mg II—to see the measured line width and continuum placement that yield MBH≈4.4×10^8 M⊙ and underpin the Eddington-ratio estimates.