Accretion-induced distortion of the polar cap magnetic field of an accreting neutron star

We compute the distorted magnetic field structure of a neutron star accreting at its poles as a function of accreted mass M-a by evolving an initial dipole through a quasistatic sequence of Grad-Shafranov equilibria. Our calculation is the first to (i) follow the development of high-order multipoles due to equatorial hydromagnetic spreading, and (ii) maintain strict flux freezing between the initial and final states. We find that one requires M-a greater than or equal to 10(-5) M-circle dot to significantly distort the field, higher than previous estimates (similar to 10(-10) M-circle dot), and that buoyant magnetic bubbles can form when the distortion is severe.