2015MNRAS.449.1422M

We have determined the relation between the AGN luminosities at rest-frame 6 µm associated with the dusty torus emission and at 2-10 keV energies using a complete, X-ray-flux-limited sample of 232 AGN drawn from the Bright Ultra-hard XMM-Newton Survey. The objects have intrinsic X-ray luminosities between 10⁴² and 10⁴⁶erg/s and redshifts from 0.05 to 2.8. The rest-frame 6 µm luminosities were computed using data from the Wide-field Infrared Survey Explorer and are based on a spectral energy distribution decomposition into AGN and galaxy emission. The best-fitting relationship for the full sample is consistent with being linear, L6µm∝L_2-10 keV^0.99±0.03, with intrinsic scatter, Δ log L6µm ∼ 0.35 dex. The L_6 µm/L_2-10 keV luminosity ratio is largely independent of the line-of-sight X-ray absorption. Assuming a constant X-ray bolometric correction, the fraction of AGN bolometric luminosity reprocessed in the mid-IR decreases weakly, if at all, with the AGN luminosity, a finding at odds with simple receding torus models. Type 2 AGN have redder mid-IR continua at rest-frame wavelengths <12 µm and are overall ∼ 1.3-2 times fainter at 6 µm than type 1 AGN at a given X-ray luminosity. Regardless of whether type 1 and type 2 AGN have the same or different nuclear dusty toroidal structures, our results imply that the AGN emission at rest-frame 6 µm is not isotropic due to self-absorption in the dusty torus, as predicted by AGN torus models. Thus, AGN surveys at rest-frame ∼ 6 µm are subject to modest dust obscuration biases.