MAGNETIC-FIELD EVOLUTION OF ACCRETING NEUTRON-STARS

We study the evolution of the magnetic field of an accreting neutron s tar in the frozen field and incompressible fluid approximations. The p lasma is accreted onto two polar caps and squeezes some of the surface material of the neutron star toward the equator. The frozen B-field i s then pushed toward the equator and is eventually buried there. The m agnetic field within the polar cap areas, which is defined by the Alfv en radius, decreases due to the expansion of the polar cap areas resul ting from the physical motion of the accreted material, which conserve s the magnetic flux. But the decrease of the magnetic field also chang es the Alfven radius which modifies the size of the polar cap and also affects the decrease of the magnetic flux within the polar caps. Ther efore, the magnetic field enclosed by the polar caps appears to decay rapidly with a time scale of similar to 10(5) m(B)/10(-3) M./M/(10(18) g s(-1)) years. As a consequence the magnetic field outside the polar cap is increasing because the total flux of the entire stellar surfac e is conserved in our approximations. The decrease of the polar cap ma gnetic field will stop and reach a minimum value similar to 10(8) G wh en the magnetic field outside the polar cap reaches B-out similar to 1 0(15)G, which is strong enough to stop the motion of the accretion mat erial across the stellar surface. However, this strong B-out cannot be observed because the accreted matter stopped by this strong field can not move toward the equator. Instead it moves inward and pulls this fi eld inside the crust with a time scale similar to 10(6) H(5)R(6)(2)rho (14)(M) over dot(18)(-1) yr. Pulsars accreting similar masses but havi ng very different magnetic field may result from different equations o f state.