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Title:The magnetospheres of accreting neutron stars
Author(s):Elsner, Ronald Fred
Doctoral Committee Chair(s):Lamb, Frederick K.
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
accreting neutron stars
star formation
accretion onto compact objects
Abstract:Accretion onto compact objects may power some compact X-ray sources. In the present study, we develop further the theory of accretion onto magnetic neutron stars in the particular case when the accretion flow is radial and the neutron star is slowly rotating. We examine in detail the formation and structure of the star's magnetosphere, the physical processes that occur in the neighborhood of the magnetospheric boundary, and the manner in which accreting plasma.enters the magnetosphere. When plasma doe$ not enter and the boundary is in static equilibrium, the boundary shape is determined by the balance between the confining pressure outside and the pressure of the stellar field inside. We illustrate the way the shape differs for different scalings of confining pressure with radius, and show that cusps form in the polar magnetosphere if the scale of the magnetosphere is much larger than the star's radius. When plasma flows across the boundary, the structure of the magnetosphere is more complex than in static eqilibrium, but in some limiting cases the boundary shape resembles that of static magnetospheres. Plasma entry when the boundary is Rayleigh-Taylor stable is examined, and, under conditions typical of bright binary X-ray sources, cusp losses and diffusion across stellar magnetic field lines are eliminated as likely ways that most of the accreting plasma might enter the magnetosphere. Reconnection of strong, large-scale magnetic fields embedded in the accreting plasma to the stellar field can keep pace with the inflow towards the magnetosphere if reconnection proceeds at its maximum theoretical rate. However, for a wide range of conditions, Rayleigh-Taylor instability of the magnetospheric boundary is likely to be the most important plasma entry process. The condition for the onset of this instability is investigated using MHD stability theory, and the nature of the unstable modes in the linear regime is studied using a linear perturbation analysis. We show that the weight of plasma on the boundary is sufficient to drive it unstable only if the ions cool to a temperature Tcrit -0.3 of their local free-fall temperature. We show that Compton scattering cools plasma bathed by X-rays from the stellar surface and that if the magnetosphere is uniformly illuminated, plasma is likely to enter via this instability over most of the boundary. Two extreme examples of possible flows within the magnetosphere, and their effects on X-ray emission from the stellar surface, are described in a qualitative way. Analytical and numerical solutions for the flow and cooling of plasma in the region between The standoff shock wave outside the magnetosphere and the magnetospheric boundary are presented for the case when the plasma there is exposed to X-rays. The applicability of the present calculations to observed compact X-ray sources and some implications for X-ray burst sources are discussed.
Issue Date:1976
Genre:Dissertation / Thesis
Rights Information:Copyright 1976 Ronald Fred Elsner
Date Available in IDEALS:2011-07-05
Identifier in Online Catalog:2007064

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