2015ApJ...801..127V

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 µm flux variations lag behind those of the optical continuum by 37.2 ± _–2.2^+2.4 days and 47.1_–3.1^+3.1 days, respectively. We report a cross-correlation time lag between the 4.5 and 3.6 µm flux of 13.9_–0.1^+0.5 days. The lags indicate that the dust emitting at 3.6 and 4.5 µm is located at a distance ≈ 1 light-month (≈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 ∼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 µm reverberation radii fall above the K-band τ ∝ L^0.5 size-luminosity relationship by factors ≲ 2.8 and ≲ 3.4, respectively, while the 4.5 µm reverberation radius is only 27% larger than the 3.6 µm radius. This is broadly consistent with clumpy torus models, in which individual optically thick clouds emit strongly over a broad wavelength range.