Revisiting the relationship between 6 μm and 2-10 keV continuum luminosities of AGN

Abstract

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 1046 erg s-1 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 ∝L2-10 keV0.99±0.03, with intrinsic scatter, Δ log L6μm ~ 0.35 dex. The L6μm/L2-10 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 themid-IR decreases weakly, if at all, with theAGNluminosity, 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.