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

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) (keV) > 2.4 x 10(-12) erg cm(-2) s(-1), drawn from the ROSAT Bright Survey catalogue. 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-x), where L-6 is corrected for the contamination of host galaxies using the spectral energy distribution fitting technique. We confirm that L-6 and L-x are correlated over four orders of magnitude, in the range of L-x = 10(42-46) erg s(-1). We investigate what kinds of physical parameters regulate this correlation. We find that L-x/L-6 clearly depends on the Eddington ratio (lambda(Edd)) as log lambda(Edd )= -(0.56 +/- 0.10) log (L-x/L-6) - (1.07 +/- 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 +/- 0.07, indicating a moderately tight correlation between L-x/L-6 and lambda(E)(dd). 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.