Hydromagnetic structure of a neutron star accreting at its polar caps

Abstract

The hydromagnetic structure of a neutron star accreting symmetrically at both magnetic poles is calculated as a function of accreted mass, Ma, starting from a polytropic sphere plus central magnetic dipole (Ma = 0) and evolving the configuration through a quasistatic sequence of two-dimensional, Grad-Shafranov equilibria as Ma increases. It is found that the accreted material spreads equatorward under its own weight, compressing the magnetic field into a thin boundary layer and burying it everywhere except in a narrow, equatorial belt. The magnetic dipole moment of the star is given by μ = 5.2 × 1024(B0/1012.5G)1.3(Ṁ a/10-8 M⊙ yr-1)0.18(Ma/M⊙) -1.3Gcm3, and the fractional difference between ..