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

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 ($0 \farcs 35$ 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 ($L_{\mathrm{MIR}}$) and 2–10 keV ($L_{\mathrm{X}}$) luminosities are correlated at a highly significant level. The best fit power-law to our data is $\log{L_{\mathrm{MIR}}} \propto \left(1.60 \pm 0.22 \right) \log{L_{\mathrm{X}}}$, showing a much smaller dispersion than earlier studies. Conclusions. The similarity in the $\log{L_{\mathrm{MIR}}}$/$\log{L_{\mathrm{X}}}$ 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.