Accretion Discs around magnetised Stars, in Particular Neutron Stars

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Addis Ababa University


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



Particular Neutron Stars