GPU-based Monte Carlo dust radiative transfer scheme applied to AGN
  A three dimensional parallel Monte Carlo (MC) dust radiative transfer code is
presented. To overcome the huge computing time requirements of MC treatments,
the computational power of vectorized hardware is used, utilizing either
multi-core computer power or graphics processing units. The approach is a
self-consistent way to solve the radiative transfer equation in arbitrary dust
configurations. The code calculates the equilibrium temperatures of two
populations of large grains and stochastic heated polycyclic aromatic
hydrocarbons (PAH). Anisotropic scattering is treated applying the
Heney-Greenstein phase function. The spectral energy distribution (SED) of the
object is derived at low spatial resolution by a photon counting procedure and
at high spatial resolution by a vectorized ray-tracer. The latter allows
computation of high signal-to-noise images of the objects at any frequencies
and arbitrary viewing angles. We test the robustness of our approach against
other radiative transfer codes. The SED and dust temperatures of one and two
dimensional benchmarks are reproduced at high precision. We utilize the
Lucy-algorithm for the optical thin case where the Poisson noise is high, the
iteration free Bjorkman & Wood method to reduce the calculation time, and the
Fleck & Canfield diffusion approximation for extreme optical thick cells. The
code is applied to model the appearance of active galactic nuclei (AGN) at
optical and infrared wavelengths. The AGN torus is clumpy and includes fluffy
composite grains of various sizes made-up of silicates and carbon. The
dependence of the SED on the number of clumps in the torus and the viewing
angle is studied. The appearance of the 10 micron silicate features in
absorption or emission is discussed. The SED of the radio loud quasar 3C 249.1
is fit by the AGN model and a cirrus component to account for the far infrared
emission.
