Oral Presentation
Discovery of an X-ray Luminous Radio-Loud Quasar at z = 3.4: A Possible Transitional Super-Eddington Phase
Presenter: Sakiko Obuchi (Waseda University)
How supermassive black holes (SMBHs) acquired their mass remains one of the fundamental open questions. Recent JWST discoveries have pushed the redshift of active galactic nuclei (AGN) up to z ≈ 11, further deepening the tension between observations and theoretical models of black hole seed formation. One possible solution is an accretion phase exceeding the classical Eddington limit, known as super-Eddington phase. In such environments, the central SMBHs are often heavily obscured by abundant gas and dust, highlighting the importance of multi-wavelength AGN surveys.
We present multi-wavelength properties of eFEDS J084222.9+001000 (eFEDS 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 L_0.5-2 keV = 10^46.20 erg/s, with a steep X-ray photon index (Γ = 2.43), and a radio counterpart detected with VLA/FIRST 1.4 GHz and VLASS 3 GHz bands. Subaru/MOIRCS J-band spectra also clearly detected MgII2800 emission, yielding the black hole mass of M_BH =(4.40±0.72)×10^8, and an Eddington ratio of λ_Edd = 1.44±0.24 from the UV continuum luminosity and λ_Edd = 12.8±3.9 from the X-ray luminosity, indicating super-Eddington accretion. This extreme X-ray luminosity results in a high α_OX = −1.20, larger than quasars and little red dots in super-Eddington phase with similar UV luminosities, typically showing α_OX < −1.8. We propose that ID830 may represent a transitional phase after an accretion burst, evolving from a super-Eddington to a sub-Eddington state, which could naturally describe its unusually high αOX.
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