Abstract
The spectral shape of an X-ray source strongly depends on the amount and distribution of the surrounding material. The spectrum of a primary source which is located in an optically thin medium with respect to Compton scattering is mainly modified by photo absorption in the lower energy range and is almost unaltered above ~ 10 keV. This picture changes when the source is obscured by gas exceeding hydrogen column densities of ~ 10<sup>24</sup> cm<sup>−2</sup>. At this degree of absorption it is likely that photons are scattered at least twice before leaving the medium. The multiple scatterings lead to a lack of photons in the high energy range of the resulting spectrum as well as to an accumulation of photons at moderate energies forming the so-called Compton-bump. The shape of the fluorescent lines also changes since scattered line photons form several Compton-shoulders which are very prominent especially for Compton-thick sources. Using a Monte Carlo method, we demonstrate the importance of Compton scattering for high column densities. For that purpose, we compare our results with existing absorption models that do not consider Compton scattering. These calculations will be implemented in a prospective version of the <em>tbabs</em> absorption model including an analytic evaluation of the strength of the fluorescent lines.
Highlights
Some astronomical X-ray sources are deeply embedded in gas exceeding hydrogen column densities of ∼ 1024 cm−2
The spectrum of a primary source which is located in an optically thin medium with respect to Compton scattering is mainly modified by photo absorption in the lower energy range and is almost unaltered above ∼ 10 keV
The flux below 10 keV is strongly reduced by photo absorption and the absorbed photons may be re-emitted as fluorescent lines
Summary
Some astronomical X-ray sources are deeply embedded in gas exceeding hydrogen column densities of ∼ 1024 cm−2 (e.g. the X-ray binary IGR J16318-4848 [1]). Compton scattering affects the spectral shape mainly due to down-scattering of high-energetic photons and has an important impact on the fluorescent line profiles. Interpreted, these modifications encode information about the composition, structure and geometrical formation of the source’s environment. Current absorption models mostly neglect the effect of Compton scattering, though it is the dominant process for dense gas and high photon energies For this reason, we present on a revised version of the tbabs [2] the absorption model which includes both Compton scattering and fluorescent line emission and is appropriate to model the spectra of highly absorbed systems
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