Accretion Discs around magnetised Stars, in Particular Neutron Stars
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Date
2010-02
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Addis Ababa University
Abstract
We develop a self-consistent theoretical model for the steady-state of an axisymmetric thin ac-
cretion disc with an internal dynamo around a magnetised stars. Starting from the vertically
integrated equations of magnetohydrodynamics we derive a single ordinary di®erential equation
for a thin accretion disc around a massive magnetic dipole and based on the analytical formu-
lation we integrate this equation numerically from the outside inwards. Our numerical solution
shows that the torque between the star and the accretion disc is dominated by the contribution
from the dynamo in the disc. We extend this model for weak magnetic ¯elds and we present
millisecond x-ray pulsars have weak magnetic dipole moments of » 1016 Tm3 compared to ordi-
nary X-ray pulsars with dipole moments of 1020 Tm3. For this reason a surrounding accretion
disc can extend closer to the neutron star, and thus reach a higher temperature, at which the
opacity is dominated by electron scattering and radiation pressure is strong. We compute the
self-similar structure of such a geometrically thin axisymmetric accretion disc with an internal
dynamo for the three regions of the disc. For the outer disc region which corresponds to the gas
pressure and Kramer's opacity dominated, in the middle region the gas pressure and electron
scattering dominated , in the inner region the radiation pressure and electron scattering dom-
inated accretion disc. Our numerical solution shows that the torque between the star and the
accretion disc is dominated by the coupling between the stellar magnetic ¯eld and the dynamo
in the disc. Finally, we have developed that the time-dependent equations for an accretion disc
and linear stability analysis of steady-state disc solutions in the presence of a strong external
magnetic ¯eld. The analytical and numerical analysis of the solutions to the stability properties
and time evolution will tell us the observed behaviours of the torque between the disc and the
star.
Key words: accretion, accretion discs - magnetohydrodynamics (MHD) - magnetic ¯elds -
stars: neutron X-rays: binaries- pulsars: general
Description
Keywords
Particular Neutron Stars