Accretion and evolution of the neutron star magnetic field

Abstract

The weak magnetic field of many old pulsars entering binary systems strongly supports the idea of the field decay in those objects. The present paper considers a mechanism for fast field decay associated with additional heating of the neutron star during the mass transfer phase. Heating decreases the conductivity of the neutron star crust and may be responsible for an acceleration of the magnetic field decay if the currents maintaining this field are concentrated in the crust. The field strength at the end of the accretion phase depends on both the total amount of the accreted mass, |$\Delta M$|⁠, and the accretion rate, M. Computations have been performed for a wide range of accretion rates and durations of the accretion phase. The dependence of the field on the accreted mass can be fitted by a simple power law, |$B\propto \Delta M^{-\gamma}$| with |$\gamma\approx0.8$|⁠, for relatively high values of |$\Delta M, \Delta M\gt10^{-6} \text M_\odot\enspace \text {yr}^{-1}$|⁠. The low magnetic field of many pulsars entering binary systems may well be accounted for by this mechanism.