The small dispersion of the mid IR -- hard X-ray correlation in AGN

Abstract

Context: We investigate mid-infrared and X-ray properties of the dusty torus in unification scenarios for active galactic nuclei. Aims: We use the relation between mid IR and hard X-ray luminosities to constrain AGN unification scenarios. Methods: With VISIR at the VLT, we have obtained the currently highest angular resolution (0".35 FWHM) narrow-band mid infrared images of the nuclei of 8 nearby Seyfert galaxies. Combining these observations with X-ray data from the literature we study the correlation between their mid IR and hard X-ray luminosities. Results: We find that the rest frame 12.3 mircon (L_MIR) and 2-10 keV (L_X) luminosities are correlated at a highly significant level. The best fit power-law to our data is log L_MIR \propto (1.60 \pm 0.22) log L_X, showing a much smaller dispersion than earlier studies. Conclusions: The similarity in the og L_MIR / log L_X ratio between Sy1s and Sy2s even using high angular resolution MIR data implies that the similarity is intrinsic to AGN and not caused by contamination from extra-nuclear emission. This supports clumpy torus models. The exponent of the correlation constrains the inner geometry of the torus.