Simple numerical X-ray polarization models of reflecting axially symmetric structures around accreting compact objects

Author:

Podgorný J123ORCID,Dovčiak M2ORCID,Marin F1ORCID

Affiliation:

1. Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg , UMR 7550, F-67000 Strasbourg , France

2. Astronomical Institute, Academy of Sciences of the Czech Republic , Boční II, CZ-14131 Prague , Czech Republic

3. Astronomical Institute, Charles University , V Holešovičkách 2, CZ-18000 Prague , Czech Republic

Abstract

ABSTRACT We present a series of numerical models suitable for X-ray polarimetry of accreting systems. First, we provide a spectropolarimetric routine that integrates reflection from inner optically thick walls of a geometrical torus of arbitrary size viewed under general inclination. In the studied example, the equatorial torus is illuminated by a central isotropic source of X-ray power-law emission, representing a hot corona. Nearly neutral reprocessing inside the walls is precomputed by Monte Carlo code stokes that incorporates both line and continuum processes, including multiple scatterings and absorption. We created a new xspec model, called xsstokes, which in this version enables efficient X-ray polarimetric fitting of the torus parameters, observer’s inclination and primary emission properties, interpolating for arbitrary state of primary polarization. Comparison of the results to a Monte Carlo simulation allowing partial transparency shows that the no-transparency condition may induce different polarization by tens of per cent. Allowing partial transparency leads to lower/higher polarization fraction, if the resulting polarization orientation is perpendicular/parallel to the rotation axis. We provide another version of xsstokes that is suitable for approximating nearly neutral reflection from a distant optically thick disc of small geometrical thickness. It assumes local illumination averaged for a selected range of incident angles, representing a toy model of a diffuse corona of various physical extent. Assessing both xsstokes variants, we conclude that the resulting polarization can be tens of per cent and perpendicularly/parallelly oriented towards the rotation axis, if the reflecting medium is rather vertically/equatorially distributed with respect to a compact central source.

Funder

Czech Science Foundation

Publisher

Oxford University Press (OUP)

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