MAGNETIC-FIELD EVOLUTION OF ACCRETING NEUTRON-STARS

Observations suggest a connection between the low magnetic fields of b inary and millisecond pulsars and their being processed in binary syst ems, indicating accretion-induced field decay in such cases. A possibl e mechanism is that of rapid ohmic decay in the accretion-heated crust . The effect of accretion on purely crustal fields, for which the curr ent loops are completely confined within the solid crust, is two-fold. On the one hand the heating reduces the electrical conductivity and c onsequently the ohmic decay time-scale, inducing a faster decay of the field. At the same time the material movement, caused by the depositi on of matter on top of the crust, pushes the original current-carrying layers into deeper and denser regions where the higher conductivity s lows the decay down. This results in a competition between these two o pposing processes. The mass of the crust of a neutron star changes ver y little with a change in the total mass; accretion therefore implies assimilation of the original crust into the superconducting core. When the original current-carrying regions undergo such assimilation, furt her decay is stopped altogether. We perform model evolutionary calcula tions for a range of values of the accretion rate and the crustal temp erature. We find that in all cases an initial phase of rapid decay is followed by a slow-down and finally a freezing of the surface field. T he pre-accretion phase of field decay in the effectively isolated neut ron star plays a significant role. In this phase the currents diffuse down through the whole of the crust by pure ohmic dissipation, and the longer it lasts the deeper the currents penetrate. If prior to the ac cretion phase the currents have already penetrated to the regions of h igh density and hence high conductivity, the effect of crustal heating is not as dramatic.