Proton deceleration near the surface on an accreting neutron star
Pakey, Donald Dean
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https://hdl.handle.net/2142/23859
Description
Title
Proton deceleration near the surface on an accreting neutron star
Author(s)
Pakey, Donald Dean
Issue Date
1990
Doctoral Committee Chair(s)
Lamb, Frederick K.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
proton deceleration
accreting neutron star
x-radiation
Coulomb collisions
Language
en
Abstract
In binary X-ray sources consisting of a neutron star accreting matter from a companion star, Xradiation
is produced by the layer of hot, dense plasma that develops where the matter collides with
the neutron star surface. The structure of this layer and the spectrum of the radiation produced there
are determined by the way in which infalling protons deposit their energy as a function of the
depth. The dominant stopping mechanism is binary Coulomb collisions with electrons in the
plasma at the surface of the neutron star, a process that is strongly modified if the neutron star has
a strong (B? 1012 gauss) magnetic field. For magnetic fields of this size, the electron motion
perpendicular to the field is quantized, collisions are anisotropic, and the protons deposit their
energy much deeper than if the magnetic field is weaker.
This thesis is a study of proton deceleration by multiple Coulomb collisions with electrons in a
hot, strongly magnetized plasma. The evolution of the proton velocity-space distribution function
is followed by integrating a Fokker-Planck equation numerically. The Fokker-Planck coefficients
are calculated for conditions in which the electrons have a thermal distribution of velocities parallel
to the field and remain in their lowest magnetic Landau levels. Nonrelativistic Landau
wavefunctions are used for both the electrons and the accreting protons. The protons are assumed
to be moving nearly parallel to the magnetic field initially.
The proton velocity distribution evolves in one of two ways, depending on the ratio of the
proton initial velocity to the thermal velocity of the electrons along the magnetic field. If the proton
initial velocity is large, the protons acquire substantial transverse velocities before being
significantly slowed and deposit most of their kinetic energy near the end of their paths deep in the
plasma at the surface of the star. If the initial proton velocity is small, on the other hand, the
protons do not acquire significant transverse velocities before stopping and deposit their energy
more uniformly over their paths. This latter evolution more closely resembles stopping in a
nonmagnetic plasma.
The significance of these results for X-ray emission by accreting neutron stars is discussed and
the reasons for the discrepancies among previous calculations is clarified.
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