Physics of Compton Scattering and X-Ray Spectra of Weakly Magnetic Accreting Neutron Stars
Psaltis, Dimitrios
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https://hdl.handle.net/2142/85193
Description
Title
Physics of Compton Scattering and X-Ray Spectra of Weakly Magnetic Accreting Neutron Stars
Author(s)
Psaltis, Dimitrios
Issue Date
1998
Doctoral Committee Chair(s)
Lamb, Frederick K.
Department of Study
Astronomy
Discipline
Astronomy
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, General
Language
eng
Abstract
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.
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