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 m 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 10 42 and 10 46 erg s -1 and redshifts from 0.05 to 2.8. The rest-frame 6 m 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, L 6 m L 0.990.03 2-10 keV , with intrinsic scatter, log L 6 m 0.35 dex. The L 6 m /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 m and are overall 1.3-2 times fainter at 6 m 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 m is not isotropic due to self-absorption in the dusty torus, as predicted by AGN torus models. Thus, AGN surveys at rest-frame 6 m are subject to modest dust obscuration biases.