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

TL;DR

ID830 is a $z=3.4351$ radio-loud quasar in the eFEDS field with an absorption-corrected rest-frame X-ray luminosity of $log L_{0.5-2keV}≈46.20$ and a steep spectrum ($Gamma≈2.43$), indicating unusually strong X-ray emission for its UV luminosity. The authors estimate a black hole mass $M_BH≈4.4e8 M_sun$ from the MgII line and derive Eddington ratios of $lambda_Edd,UV≈1.44$ and $lambda_Edd,X≈12.8$, pointing to super-Eddington accretion. The UV-to-X-ray ratio is high, with $alpha_OX≈-1.20$ (or $-1.42$ after correcting for jet-related X-ray excess), suggesting coexistence of a radio jet and X-ray corona during a high accretion phase. They propose ID830 as a transitional object evolving from super-Eddington to sub-Eddington after an accretion burst, with implications for SMBH growth, jet–corona feedback, and selection biases against dusty, high-z AGN. The study demonstrates the power of combining X-ray and radio selection to identify extreme, potentially obscured or beamed AGN at $z>3$ and to probe early SMBH growth.

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 $α_\mathrm{OX}$.

Figures (2)

• Figure 1: Absorption-corrected 0.5--2 keV Luminosity of the radio AGN sample in Ichikawa2023 as a function of redshift (gray circles). The sample shows selected 113 sources with reliable spec-$z$ and near-IR detections. ID830 is enclosed by the open red star. The orange shaded area represents $L_\mathrm{0.5-2\,keV} > 10^{46}$ erg s$^{-1}$, where only ID830 resides in this region.
• Figure 2: The X-ray spectrum in the observed frame of ID830 fitted by Xspec. The cross plots represent the observed data of ID830. The solid line is the best-fit spectrum, including the AGN model (dotted line) and the background model (dashed line). The fitting residuals of the best-fit model divided by the $1\sigma$ errors are shown in the lower panel.