Burial of the polar magnetic field of an accreting neutron star - I. Self-consistent analytic and numerical equilibria

Abstract

The hydromagnetic structure of a neutron star accreting symmetrically at both magnetic poles is calculated as a function of accreted mass, Ma, and polar cap radius, starting from a centred magnetic dipole and evolving through a quasi-static sequence of two-dimensional, Grad-Shafranov equilibria. The calculation is the first to track fully the growth of high-order magnetic multipoles, due to equatorward hydromagnetic spreading, while simultaneously preserving flux-freezing and a self-consistent mass-flux distribution. Equilibria are constructed numerically by an iterative scheme and analytically by Green functions. Two key results are obtained, with implications for recycled pulsars, (i) The ..