NONSTEADY STATE ACCRETION AND EVOLUTION OF HER X-1 LIKE SYSTEMS

The magnetic evolution of pulsars in binary systems is strongly influe nced by accretion. Accretion-induced heating of the neutron star inter ior decreases the crustal conductivity and results in a relatively rap id magnetic field decay if the field is maintained by currents in the crust. The surface field strength is determined by both the accretion rate and the duration of the accretion phase (or, in other words, the total amount of accreted mass). In the present paper, we consider the magnetic evolution of the neutron star at the beginning of the accreti on phase when neither the accretion rate nor the internal temperature has reached a steady state. Our calculations show that the held can be reduced by only a small factor during the first 10(4)-10(5) yr after the onset of the accretion in spite of a high accretion rate in this p eriod, provided that the currents supporting the field structure are c oncentrated in the deep crustal layers. This result can account for th e strong observable magnetic fields (greater than or equal to 10(12) G ) of pulsars in Her X-1 like systems where the accretion rate is as hi gh as 10(-8)-10(-9) M. yr(-1). In these systems accretion probably sta rted not long ago, but the pulsar is old enough to allow for a diffusi on of the field down to the crust-core boundary.