Monte Carlo simulations of X-ray absorption in the interstellar medium

Abstract

Although prior measurements of X-ray radiation from the sun exist, the first detection of a cosmical X-ray source in 1962 was still a surprise. It turned out that the signal cames from the binary system Scorpius X-1. Still a long time after this discovery, the physical process leading to the observed luminosities was mysterious. Several years later, the idea that the radiation originates from mass accretion onto a compact object, for example, a neutron star or a black hole, became general accepted. This is the case in X-ray binaries: they consist of a compact object and a normal star orbiting around their common center of mass. If the compact object orbits close to its companion star, mass accretion becomes possible. The systems are often deep embedded in sourrounding interstellar medium. As the photons interact with the particles via scattering and photo-ionization, the primary spectrum is significantly modified after it has passed the medium. In this thesis, a Monte Carlo simulation of radiative transfer in the interstellar medium is presented. After the basic idea behind the Monte Carlo method is described in the introduction, the physical background of the involved process are outlined in chapter 2. The general algorithm is described in detail in chapter 3, with special emphasis on the used Monte Carlo technique. The shape of the resulting spectra is strongly dependent on the extension and properties of the interstellar medium. Therefore, in chapter 4, spectra are shown produced within different geometries. The results are compared with prior models. The shape and intensity of the fluorescence lines is an important diagnostic tool to determine the properties of the interstellar medium around X-ray sources. For this reason, different parameters characterizing the iron K α-line are calculated as a function of degree of absorption, viewing angle and iron abundance. The resulting values may help to interpret observational data.