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 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 $\log (L_{0.5-2\mathrm{keV}}/\mathrm{erg}{\mathrm{s}}^{-1})=46.20\pm 0.12$, with a steep X-ray photon index ($\Gamma =2.43\pm 0.21$), 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$\lambda$2800 emission line width, and an Eddington ratio from the dust-extinction-corrected UV continuum luminosity reaches ${\lambda }_{\mathrm{Edd},\mathrm{UV}}=1.44\pm 0.24$ and ${\lambda }_{\mathrm{Edd},\mathrm{X}}=12.8\pm 3.9$ from the X-ray luminosity, both indicating the super-Eddington accretion. ID830 shows a high ratio of UV-to-X-ray luminosities, ${\alpha }_{\mathrm{OX}}=-1.20\pm 0.07$ (or ${\alpha }_{\mathrm{OX}}=-1.42\pm 0.07$ after correcting for jet-linked X-ray excess), higher than quasars and little red dots in super-Eddington phase with similar UV luminosities, with ${\alpha }_{\mathrm{OX}}<-1.8$. Such a high ${\alpha }_{\mathrm{OX}}$ 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 ${\alpha }_{\mathrm{OX}}$.

20 pages, 11 figures, accepted for publication in ApJ