Evolution of the magnetic fields of neutron stars in low-mass binary systems

Abstract

We investigate the evolution of magnetic fields of neutron stars in 'wide' low-mass binary systems due to the spin-down of the neutron star, resulting from an interaction with the stellar wind of the companion. We assume that magnetic flux from the neutron star core is expelled to the crust as the neutron star spins down due to the proposed vortex-fluxoid pinning interaction in the superfluid interior (Srinivasan et al. 1990). Once deposited in the crust the field will then decay due to ohmic dissipation. We construct models with a range of donor masses, orbital periods, mass-loss rates, and assumed ohmic decay timescales in the crust, and compute the final magnetic field strengths of the neutron stars. We find that the magnetic fields of millisecond pulsars, as.welI as other low-mass binary pulsars may be well accounted for by this mechanism if the ohmic decay timescale at the bottom of a neutron star crust is ~ 10^9 years. The models seem to further indicate that an asymptotic value ~ 10^8G is the lowest possible field strength obtainable by this mechanism.