Modeling optical and UV polarization of AGNs. I. Imprints of individual scattering regions

Context: Spectropolarimetry of AGNs is a powerful tool for studying the structure and kinematics of the inner regions of quasars.
Aims: We wish to investigate the effects of various AGN scattering region geometries on the polarized flux.
Methods: We introduce a new, publicly available Monte Carlo radiative transfer code, stokes, which models polarization induced by scattering off free electrons and dust grains. We model a variety of regions in AGNs.
Results: We find that the shape of the funnel of the dusty torus has a significant impact on the polarization efficiency. A compact torus with a steep inner surface scatters more light toward type-2 viewing angles than a large torus of the same half-opening angle, θ_0. For θ₀ < 53 °, the scattered light is polarized perpendicularly to the symmetry axis, whilst for θ₀ > 60 ° it is polarized parallel to the symmetry axis. In between these intervals the orientation of the polarization depends on the viewing angle. The degree of polarization ranges between 0% and 20% and is wavelength independent for a large range of θ_0. Observed wavelength-independent optical and near-UV polarization thus does not necessarily imply electron scattering. Spectropolarimetry at rest-frame wavelengths less than 2500 Å may distinguish between dust and electron scattering but is not conclusive in all cases. For polar dust, scattering spectra are reddened for type-1 viewing angles, and made bluer for type-2 viewing angles. Polar electron-scattering cones are very efficient polarizers at type-2 viewing angles, whilst the polarized flux of the torus is weak.
Conclusions: .We predict that the net polarization of Seyfert-2 galaxies decreases with luminosity, and conclude that the degree of polarization should be correlated with the relative strength of the thermal IR flux. We find that a flattened, equatorial, electron-scattering disk, of relatively low optical depth, reproduces type-1 polarization. This is insensitive to the exact geometry, but the observed polarization requires a limited range of optical depth.