2019MNRAS.484..196T

We revisit the correlation between the mid-infrared (6 µm) 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 ×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 µm luminosity (L₆) and the rest-frame 2-10 keV luminosity (L_X), where L₆ is corrected for the contamination of host galaxies using the spectral energy distribution fitting technique. We confirm that L₆ 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/L₆ clearly depends on the Eddington ratio (λ_Edd) as logλ_ 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 L₆ > 10⁴⁶ 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/L₆ and λ_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.