Does the mid-infrared-hard X-ray luminosity relation for active galactic nuclei depend on Eddington ratio?

Abstract

We revisit the correlation between the mid-infrared (6 $\mu$m) and hard X-ray (2--10 keV) luminosities of active galactic nuclei (AGNs) to understand the physics behind it. We construct an X-ray flux-limited sample of 571 type 1 AGNs with $f_{0.5-2.0\mathrm{keV}}>2.4\times 10^{-12}$ erg cm${}^{-2}$ s${}^{-1}$, drawn from the ROSAT Bright Survey catalog. Cross-matching the sample with infrared data taken from Wide-field Infrared Survey Explorer, we investigate the relation between the rest-frame 6 $\mu$m luminosity ($L_6$) and the rest-frame 2--10 keV luminosity ($L_{\mathrm{X}}$), where $L_6$ is corrected for the contamination of host galaxies by using the spectral energy distribution fitting technique. We confirm that $L_6$ and $L_{\mathrm{X}}$ are correlated over four orders of magnitude, in the range of $L_{\mathrm{X}}=10^{42-46}$ erg s${}^{-1}$. We investigate what kinds of physical parameters regulate this correlation. We find that $L_{\mathrm{X}}$/$L_6$ clearly depends on the Eddington ratio (${\lambda }_{\mathrm{Edd}}$) as $\log {\lambda }_{\mathrm{Edd}}=-(0.56\pm 0.10)\log (L_{\mathrm{X}}/L_6)-(1.07\pm 0.05)$, even taking into account quasars that are undetected by ROSAT as well as those detected by XMM-Newton in the literature. We also add hyper-luminous quasars with $L_6$ $>$ 10${}^{46}$ erg s${}^{-1}$ in the literature and perform a correlation analysis. The resultant correlation coefficient is $-0.41\pm 0.07$, indicating a moderately tight correlation between $L_{\mathrm{X}}$/$L_6$ and ${\lambda }_{\mathrm{Edd}}$. This means that AGNs with high Eddington ratios tend to have lower X-ray luminosities with respect to the mid-infrared luminosities. This dependence can be interpreted as a change in the structure of the accretion flow.

9 pages, 4 figures, and 2 tables, accepted for publication in MNRAS

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