Constraining properties of dusty environments by infrared variability
  We present model simulations of time-variable infrared (IR) emission from
dust as a consequence of variability of the incident radiation. For that we
introduce a generalized treatment for temperature variations in a dusty
environment, which is not limited to any specific astronomical source. The
treatment has been incorporated into a simplified clumpy torus model, with the
radial brightness distribution as the main parameter, to study the IR emission
of type 1 active galactic nuclei (AGN). We show that any variability signal in
the optical is smoothened stronger if the brightness distribution is very
extended, and this smoothing strongly depends on wavelength. This also affects
time lags between the optical and near-/mid-IR emission, which can be up to 10s
of sublimation radii for long wavelengths and extended brightness
distributions. The dependence of time lag on wavelength and distribution can be
used to quantify the brightness distribution in an AGN torus, either by
comparing optical light curves to near-IR and mid-IR light curves, or by
directly comparing near-IR to mid-IR light curves. Moreover, our model has been
applied to near-IR data of the nearby Seyfert 1 galaxy NGC 4151. We show that
the simple model can reproduce the overall observed variability signal and
found that about 40% of the energy in the variability signal in the V-band has
been converted into K-band variability. This low value may be explained by a
"snowball" model of gradually-sublimating clouds at the inner edge of the
torus. We also note that our modeling does not support a change of time
lag/sublimation radius over the observed light curve epoch in spite of a
significant change in V-band emission.
