The small dispersion of the mid IR - hard X-ray correlation in active galactic nuclei

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 mu m (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 proportional to (1.60 +/- 0.22) log L-X, showing a much smaller dispersion than earlier studies. Conclusions. The similarity in the log 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.