On the Observational Difference Between the Accretion Disk-Corona Connections among Super- and Sub-Eddington Accreting Active Galactic Nuclei

Abstract

We present a systematic X-ray and multiwavelength study of a sample of 47 active galactic nuclei (AGNs) with reverberation-mapping measurements. This sample includes 21 super-Eddington accreting AGNs and 26 sub-Eddington accreting AGNs. Using high-state observations with simultaneous X-ray and UV/optical measurements, we investigate whether super-Eddington accreting AGNs exhibit different accretion disk-corona connections compared to sub-Eddington accreting AGNs. We find tight correlations between the X-ray-to-UV/optical spectral slope parameter (${\alpha }_{\mathrm{OX}}$) and the monochromatic luminosity at $2500\mathring{A}$ ($L_{2500\mathring{\mathrm{A}}}$) for both the super- and sub-Eddington subsamples. The best-fit ${\alpha }_{\mathrm{OX}}-L_{2500\mathring{\mathrm{A}}}$ relations are consistent overall, indicating that super-Eddington accreting AGNs are not particularly X-ray weak in general compared to sub-Eddington accreting AGNs. We find dependences of ${\alpha }_{\mathrm{OX}}$ on both the Eddington ratio ($L_{\mathrm{Bol}}/L_{\mathrm{Edd}}$) and black hole mass ($M_{\mathrm{BH}}$) parameters for our full sample. A multi-variate linear regression analysis yields ${\alpha }_{\mathrm{OX}}=-0.13\log (L_{\mathrm{Bol}}/L_{\mathrm{Edd}})-0.10\log M_{\mathrm{BH}}-0.69$, with a scatter similar to that of the ${\alpha }_{\mathrm{OX}}-L_{2500\mathring{\mathrm{A}}}$ relation. The hard (rest-frame $>2\mathrm{keV}$) X-ray photon index ($\Gamma$) is strongly correlated with $L_{\mathrm{Bol}}/L_{\mathrm{Edd}}$ for the full sample and the super-Eddington subsample, but these two parameters are not significantly correlated for the sub-Eddington subsample. A fraction of super-Eddington accreting AGNs show strong X-ray variability, probably due to small-scale gas absorption, and we highlight the importance of employing high-state (intrinsic) X-ray radiation to study the accretion disk-corona connections in AGNs.

31 pages, 11 figures, accepted for publication in ApJ

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