We present results from a fifteen-month campaign of high-cadence ( \sim 3 days ) mid-infrared Spitzer and optical ( B and V ) monitoring of the Seyfert 1 galaxy NGC 6418 , with the objective of determining the characteristic size of the dusty torus in this active galactic nucleus ( AGN ) .
We find that the 3.6 \mu m and 4.5 \mu m flux variations lag behind those of the optical continuum by 37.2 ^ { +2.4 } _ { -2.2 } days and 47.1 ^ { +3.1 } _ { -3.1 } days , respectively .
We report a cross-correlation time lag between the 4.5 \mu m and 3.6 \mu m flux of 13.9 ^ { +0.5 } _ { -0.1 } days .
The lags indicate that the dust emitting at 3.6 \mu m and 4.5 \mu m is located at a distance \approx 1 light-month ( \approx 0.03 pc ) from the source of the AGN UV–optical continuum .
The reverberation radii are consistent with the inferred lower limit to the sublimation radius for pure graphite grains at 1800 K , but smaller by a factor of \sim 2 than the corresponding lower limit for silicate grains ; this is similar to what has been found for near-infrared ( K-band ) lags in other AGN .
The 3.6 and 4.5 \mu m reverberation radii fall above the K-band \tau \propto L ^ { 0.5 } size-luminosity relationship by factors \lesssim 2.7 and \lesssim 3.4 , respectively , while the 4.5 \mu m reverberation radius is only 27 % larger than the 3.6 \mu m radius .
This is broadly consistent with clumpy torus models , in which individual optically thick clouds emit strongly over a broad wavelength range .