Magnetospheres of neutron stars accreting from disks
Zylstra, Gregory Jay
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https://hdl.handle.net/2142/23908
Description
Title
Magnetospheres of neutron stars accreting from disks
Author(s)
Zylstra, Gregory Jay
Issue Date
1988
Doctoral Committee Chair(s)
Lamb, Frederick K.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
magnetospheres
neutron stars
thin plasma disk
accretion-powered pulsars
quasi-periodic oscillations
Language
en
Abstract
This thesis presents quantitative models of the magnetosphere of a magnetic star surrounded by a thin plasma disk. The model is motivated by the currently accepted picture of many accretion-powered pulsars as strongly magnetic neutron stars accreting from disks. It may also be relevant to the luminous galactic bulge X-ray sources that display quasi-periodic oscillations in intensity. These QPO sources are thought by some to be weakly magnetic neutron stars accreting from disks.
In constructing these magnetospheric models, I assume that the magnetic field is static and axisymmetric about the axis of the disk. I also assume that the magnetic field is in force-free equilibrium. The validity of these simplifying assumptions for astrophysical systems is discussed in some detail.
The boundary conditions on the magnetic field at the disk have been chosen to mimic the expected electrodynamic interaction between a magnetic neutron star and an accretion disk. These boundary conditions cause the second-order partial differential equation for the magnetic flux function to be non-linear. This equation is solved iteratively using finite-difference techniques on a grid of points covering the volume of the magnetosphere.
Solutions have been obtained for two different configurations. In one, the disk extends to the stellar surface while in the other, there is a gap between the inner edge of the disk and the surface of the star. The solutions determine the poloidal
and toroidal components of the magnetic field as a function of position within the magnetosphere. From these components, I calculate the torque exerted on the star and the energy stored in the twisted magnetic field.
The significance of these solutions for accretion-powered pulsars and QPO sources is briefly considered. Specifically, I discuss the likely effect of the electrodynamic interaction between the disk and the star on the structure of the magnetic field, especially near the disk's inner edge. I also discuss the maximum pitch and largest spin-up and spin-down torques that can be supported by the magnetic field.
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