A study of electron-positron pair equilibria in models of compact X- and gamma-ray sources
Bjornsson, Gunnlaugur
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https://hdl.handle.net/2142/21895
Description
Title
A study of electron-positron pair equilibria in models of compact X- and gamma-ray sources
Author(s)
Bjornsson, Gunnlaugur
Issue Date
1990
Doctoral Committee Chair(s)
Truran, James W.
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, Astronomy and Astrophysics
Language
eng
Abstract
This thesis studies thermal electron-positron pair equilibria in two temperature models of compact $X$- and $\gamma$-ray sources. The radiation field is taken to be Comptonized bremsstrahlung. The pairs are assumed to be heated by Coulomb interaction with the much hotter protons and to cool by bremsstrahlung emission, Compton scattering and annihilation. Two parameters, the 'proton optical depth' and the compactness, characterize each equilibrium state.
First the stability of the pair equilibria in a spherical plasma cloud is considered. It is shown that a careful account of the energy balance is very important when the stability properties are determined. The equilibria are found to be unstable in a very limited range of compactness and proton optical depth. It is concluded that this particular instability is unlikely to be the cause of the observed variability of the compact sources. The same result also implies that it is possible to build up high pair densities by a thermal mechanism in two temperature environments.
The most important result of this thesis considers the effects of pairs on the structure of geometrically and effectively optically thin accretion disks. A new approach for solving for the equilibrium structure of the disks is presented. It takes full advantage of the simplicity of the disk structure equations and reduces the problem to that of considerations of the microphysics only. In effect, the pair equilibrium states are projected into the space spanned by the disk structure parameters. This allows a direct visualization of all possible disk solutions at once. Each solution profile needs to be calculated only once and a complete disk solution is obtained by a simple radial coordinate transformation. The disk solutions are thus seen to be scale free in terms of the radial coordinate as well as in terms of the mass of the central object and the accretion rate. Two particular disk solutions obtained in this way are given. It is shown that including electron-positron pairs in the disk structure calculations leads to a breakdown of the thin disk assumptions and that a more detailed disk modeling is required before electron-positron pairs can be self-consistently included.
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