Accretion Discs Around Magnetised Stars In Particular Neutron Stars

We develop a self-consistent theoretical model for the steady-state of an axisymmetric thin ac-rncretion disc with an internal dynamo around a magnetised stars. Starting from the verticallyrnintegrated equations of magnetohydrodynamics we derive a single ordinary di®erential equationrnfor a thin accretion disc around a massive magnetic dipole and based on the analytical formu-rnlation we integrate this equation numerically from the outside inwards. Our numerical solutionrnshows that the torque between the star and the accretion disc is dominated by the contributionrnfrom the dynamo in the disc. We extend this model for weak magnetic ¯elds and we presentrnmillisecond x-ray pulsars have weak magnetic dipole moments of » 1016 Tm3 compared to ordi-rnnary X-ray pulsars with dipole moments of 1020 Tm3. For this reason a surrounding accretionrndisc can extend closer to the neutron star, and thus reach a higher temperature, at which thernopacity is dominated by electron scattering and radiation pressure is strong. We compute thernself-similar structure of such a geometrically thin axisymmetric accretion disc with an internalrndynamo for the three regions of the disc. For the outer disc region which corresponds to the gasrnpressure and Kramer's opacity dominated, in the middle region the gas pressure and electronrnscattering dominated , in the inner region the radiation pressure and electron scattering dom-rninated accretion disc. Our numerical solution shows that the torque between the star and thernaccretion disc is dominated by the coupling between the stellar magnetic ¯eld and the dynamornin the disc. Finally, we have developed that the time-dependent equations for an accretion discrnand linear stability analysis of steady-state disc solutions in the presence of a strong externalrnmagnetic ¯eld. The analytical and numerical analysis of the solutions to the stability propertiesrnand time evolution will tell us the observed behaviours of the torque between the disc and thernstar.rnKey words: accretion, accretion discs - magnetohydrodynamics (MHD) - magnetic ¯elds -rnstars: neutron X-rays: binaries- pulsars: general

Get Full Work

Report copyright infringement or plagiarism