Diamagnetic Screening of the Magnetic Field in Accreting Neutron Stars

A possible mechanism for screening of the surface magnetic field of an accreting neutron star, by the accreted material, is investigated. We model the material flow in the surface layers of the star by an assumed 2-D velocity field satisfying all the physical requirements. Using this model velocity we find that, in the absence of magnetic buoyancy, the surface field is screened (submergence of the field by advection) within the time scale of material flow of the top layers. On the other hand, if magnetic buoyancy is present, the screening happens over a time scale which is characteristic of the slower flow of the deeper (hence, denser) layers. For accreting neutron stars, this longer time scale turns out to be about $10^5$ years, which is of the similar order as the accretion time scale of most massive X-ray binaries.