Masters research project: accretion onto neutron stars

New born neutron stars have very strong magnetic fields in the range 10^12 to 10^15 gauss (10^8 to 10^11 tesla). Often, neutron stars are found in binary systems where they accrete material from a companion star, emitting much of the large release of gravitational energy as X rays. These stars are found to have much weaker magnetic fields, in the range 10^8 to 10^9 gauss. The mechanism for reducing the strength of the magnetic field by such a large factor is unknown, but there are several candidates. According to the 'magnetic burial' hypothesis, the magnetic field lines are simply buried by the weight of the accreted material, which lands at the magnetic poles and spreads out horizontally, and thus hidden from view. This project will investigate how this horizontal spreading affects the existing magnetic field, looking at possible instabilities which may allow the magnetic field lines to pop back upwards through the accreted layer. This will involve a mix of analytic and numerical methods. Alternative hypotheses will also be looked at, such as diffusive processes in the interior of the star, a phase change to exotic matter, and interaction of flux and vortex tubes in a superconducting superfluid. Comparison to accreting white dwarfs should be informative.