Modeling the Infrared Reverberation Response of the Circumnuclear Dusty
  Torus in AGN: The Effects of Cloud Orientation and Anisotropic Illumination
  The obscuring circumnuclear torus of dusty molecular gas is one of the major
components of active galactic nuclei (AGN). The torus can be studied by
analyzing the time response of its infrared (IR) dust emission to variations in
the AGN continuum luminosity, a technique known as reverberation mapping. The
IR response is the convolution of the AGN ultraviolet/optical light curve with
a transfer function that contains information about the size, geometry, and
structure of the torus. Here, we describe a new computer model that simulates
the reverberation response of a clumpy torus. Given an input optical light
curve, the code computes the emission of a 3D ensemble of dust clouds as a
function of time at selected IR wavelengths, taking into account light travel
delays. We present simulated dust emission responses at 3.6, 4.5, and 30 $\mu$m
that explore the effects of various geometrical and structural properties, dust
cloud orientation, and anisotropy of the illuminating radiation field. We also
briefly explore the effects of cloud shadowing (clouds are shielded from the
AGN continuum source). Example synthetic light curves have also been generated,
using the observed optical light curve of the Seyfert 1 galaxy NGC 6418 as the
input. The torus response is strongly wavelength-dependent, due to the gradient
in cloud surface temperature within the torus, and because the cloud emission
is strongly anisotropic at shorter wavelengths. Anisotropic illumination of the
torus also significantly modifies the torus response, reducing the lag between
the IR and optical variations.
