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A. Cumming (ITP, UC Santa Barbara), E. G. Zweibel (JILA, U. Colorado, Boulder), L. Bildsten (ITP, UC Santa Barbara)
We investigate the magnetic screening properties of matter accreted onto a neutron star. Our calculation includes the unburned atmosphere of hydrogen/helium, the liquid layer of ashes and the outer crust. We compare the timescales for ohmic diffusion and thermomagnetic drift with the radial flow time due to accretion. For the highest accretion rates, the diffusion and drift times are always longer than the accretion time, making it difficult for magnetic flux to penetrate the freshly accreted material. However, for the lowest accretion rates there is adequate time for any underlying magnetic field to penetrate the accreted matter. We show the one-dimensional steady-state magnetic field profiles, for which downwards advection and compression by accretion are balanced by upwards ohmic diffusion and thermomagnetic drift, and investigate whether these profiles are stable to buoyancy instabilities. We discuss possible implications of our results for the observed magnetic fields of both steadily and transiently accreting neutron stars.