Letter to the Editor

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

¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile e-mail: hhorst@eso.org
² Zentrum für Astronomie, ITA, Universität Heidelberg, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
³ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁴ Steward Observatory, The University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA

Received: 13 June 2006
Accepted: 27 July 2006

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 ( 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 m () and 2–10 keV () luminosities are correlated at a highly significant level. The best fit power-law to our data is , showing a much smaller dispersion than earlier studies.

Conclusions.The similarity in the / 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.