The mid IR – hard X-ray correlation in AGN and its implications for dusty torus models

Abstract

Aims. We investigate mid-infrared and X-ray properties of the dusty torus invoked in the unification scenario for active galactic nuclei by using the relation between mid IR and hard X-ray luminosities to constrain the geometry and physical state of the dusty torus. Methods. We present new VISIR observations of 17 nearby AGN and combined these with our earlier VISIR sample of 8 Seyfert galaxies. After combining these observations with X-ray data from the literature, we studied the correlation between their mid IR and hard X-ray luminosities. Results. A statistically highly significant correlation is found between the rest frame 12.3 μm (L MIR ) and 2-10 keV (L x ) luminosities. Furthermore, with a probability of 97%, we find that Sy 1 and Sy 2 nuclei have the same distribution of L MIR over L x . Conclusions. The high resolution of our MIR imaging allows us to exclude any significant non-torus contribution to the AGN mid IR continuum, thereby implying that the similarity in the L MIR /L X ratio between Sy 1s and Sy 2s is intrinsic to AGN. We argue that this is best explained by clumpy torus models. The slope of the correlation is in good agreement with the expectations from the unified scenario and indicates little to no change in the torus geometry with luminosity. In addition, we demonstrate that the high angular resolution is crucial for AGN studies in the IR regime.