Revisiting the relationship between 6 {\mu}m and 2-10 keV continuum luminosities of AGN

TL;DR

This study investigates the relationship between mid-infrared and X-ray luminosities in AGN, finding a nearly linear correlation that is largely unaffected by X-ray absorption, challenging simple torus models.

Contribution

It provides a comprehensive analysis of the 6 μm and 2-10 keV luminosity relation in a large AGN sample, revealing the near-linear correlation and implications for AGN unification models.

Findings

The 6 μm luminosity scales linearly with 2-10 keV luminosity (L_6μm ∝ L_2-10 keV^0.99).

The luminosity ratio is largely independent of X-ray absorption.

Type 2 AGN are 1.3-2 times fainter at 6 μm than type 1 AGN at the same X-ray luminosity.

Abstract

We have determined the relation between the AGN luminosities at rest-frame 6 {\mu}m associated to 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 and redshifts from 0.05 to 2.8. The rest-frame 6 {\mu}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-fit relationship for the full sample is consistent with being linear, L_6 {\mu}m $\propto$ L_2-10 keV^0.99$\pm$0.032, with intrinsic scatter, ~0.35 dex in log L_6 {\mu}m. The L_6 {\mu}m/L_2-10 keV luminosity ratio is largely independent on 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 {\mu}m and are overall ~1.3-2 times fainter at 6 {\mu}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 {\mu}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 {\mu}m are subject to modest dust obscuration biases.