20 - The polarization of complex X-ray sources

Summary

The radiative transfer code STOKES was extended to allow for X-ray polarimetry modelling. The physical mechanisms of Compton scattering, photo-absorption, and the production of iron K lines were added and are illustrated by modelling the X-ray polarization spectrum of irradiated, cold matter disks. These models confirm that the orientation of the polarization position angle is related to the size of the disk. Although strongly diminishing the spectral flux, an obscuring torus around a small irradiated disk significantly increases the polarization at intermediate viewing angles. Our modelling shows that the polarization can be very sensitive to the radiative coupling between different reprocessing regions.

Introduction

Polarimetry and spectropolarimetry are an important extension of photometry and spectroscopy techniques. In addition to the spectral intensity, i.e. the first Stokes parameter, the linear polarization percentage, P, and position angle, ψ, can provide further information about the geometry and the dynamics of a given object. To decode this information, accurate modelling is necessary. For this purpose I started the development of the radiative transfer code STOKES that is based on the Monte-Carlo method and that computes the polarization spectrum due to multiple photon-matter interactions in geometrically complex environments. The code is publicly available on the web. New versions of STOKES will be subsequently provided for download after they have been carefully tested and documented. The code has recently been extended to include polarization effects that are relevant in the X-ray range.