SPITZER SPACE TELESCOPE MEASUREMENTS OF DUST REVERBERATION LAGS IN THE SEYFERT 1 GALAXY NGC 6418

ABSTRACT We present results from a 15 month campaign of high-cadence (∼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 and 4.5 m flux variations lag behind those of the optical continuum by days and days, respectively. We report a cross-correlation time lag between the 4.5 and 3.6 m flux of days. The lags indicate that the dust emitting at 3.6 and 4.5 m is located at a distance light-month ( ) 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 ∼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 AGNs. The 3.6 and 4.5 m reverberation radii fall above the K-band size-luminosity relationship by factors and , respectively, while the 4.5 m reverberation radius is only 27% larger than the 3.6 m radius. This is broadly consistent with clumpy torus models, in which individual optically thick clouds emit strongly over a broad wavelength range.