Physics of Compton Scattering and X-Ray Spectra of Weakly Magnetic Accreting Neutron Stars

Compton scattering of photons by non-relativistic electrons is thought to play an important role in forming the radiation spectrum of many astrophysical systems. Here we study in detail Compton scattering of photons by electrons with non-negligible thermal and bulk velocities and apply our results to understanding the X-ray spectra of weakly magnetic accreting neutron stars. We first derive the time-dependent radiative transfer equation that describes spontaneous and induced Compton scattering as well as absorption and emission by static and moving media. We demonstrate that terms in the radiative transfer equation that are second-order in the electron bulk velocity should usually be retained, because the effects described by these terms can be as important as the effects described by the terms that are first-order in the electron bulk velocity, even when this bulk velocity is small. We then develop a numerical algorithm for solving the radiative transfer equation in systems with spherical symmetry. The algorithm is a generalization of the method of Eddington factors and provides solutions that are valid both in the diffusion and free-streaming regimes. Using this algorithm we perform detailed numerical calculations of the X-ray spectra of the so-called Z sources that are consistent with models of their rapid variability. We show that electron cyclotron emission near the surface of neutron stars with surface magnetic field strengths of 109--10 10 G, when Comptonized by a hot central corona and a cool radial inflow, produce X-ray spectra and color-color tracks in excellent agreement with observations. We suggest that the differences in the spectra and rapid variability of different Z sources can be attributed to differences in their magnetic field strengths.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Astronomy

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2015

Author dc:creator

• Psaltis, Dimitrios

Contributors dc:contributor

• Lamb, Frederick K.

• Physics, General

Language dc:language

eng

Identifier

(MiAaPQ)AAI9912344

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/85193