Volume 548, December 2012
|Number of page(s)||25|
|Published online||04 December 2012|
Modeling optical and UV polarization of AGNs
II. Polarization imaging and complex reprocessing
Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS,
UMR 7550, 11 rue de l’Université,
2 Centro de Astrofísica de Valparaíso y Departamento de Física y Astronomía, Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile
3 Astronomical Institute of the Academy of Sciences, Bočni II 1401, 14131 Prague, Czech Republic
Accepted: 12 September 2012
Context. The innermost parts of active galactic nuclei (AGNs) are believed to be comprised of several emission and scattering media coupled by radiative processes. These regions generally cannot be spatially resolved. Spectropolarimetric observations give important information about the reprocessing geometry.
Aims. We aim to obtain a coherent model of the polarization signature resulting from the radiative coupling between the components, to compare our results with polarimetry of thermal AGNs, and thereby to put constraints on the geometry.
Methods. We used a new public version of stokes, a Monte Carlo radiative transfer code presented in the first paper of this series. The code has been significantly improved for computational speed and polarization imaging has been implemented. The imaging capability helps to improve understanding of the contributions of different components to the spatially-integrated flux. We coupled continuum sources with a variety of reprocessing regions such as equatorial scattering regions, polar outflows, and toroidal obscuring dust and studied the resulting polarization. We explored combinations and computed a grid of thermal AGN models for different half-opening angles of the torus and polar winds. We also considered a range of optical depths for equatorial and polar electron scattering and investigated how the model geometry influences the type-1/type-2 polarization dichotomy for thermal AGNs (type-1 AGNs tend to be polarized parallel to the axis of the torus while type-2 AGNs tend to be polarized perpendicular to it).
Results. We put new constrains on the inflowing medium within the inner walls of the torus. To reproduce the observed polarization in type-1 objects, the inflow should be confined to the common equatorial plane of the torus and the accretion disk and have a radial optical depth of 1 < τ < 3. Our modeling of type-1 AGNs indicates that the torus is more likely to have a large (~60°) half-opening angle. Polarization perpendicular to the axis of the torus may arise at a type-1 viewing angle for a torus half-opening angle of 30°–45° or polar outflows with an optical depth near unity. Our modeling suggests that most Seyfert-2 AGN must have a half-opening angle >60° to match the expected level of perpendicular polarization. If outflows are collimated by the torus inner walls, they must not be optically thick (τ < 1) to preserve the polarization dichotomy. Then, varying the wind’s optical depth does not affect the degree of polarization of type-2 thermal AGNs but it has a significant impact on the type-1/type-2 polarization dichotomy when τ > 0.3.
Key words: galaxies: active / galaxies: Seyfert / polarization / radiative transfer / scattering
© ESO, 2012
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