Does mass accretion lead to field decay in neutron stars?

WHETHER or not neutron-star magnetic fields decay is a matter of current debate. The recent observation1,2 of cyclotron lines from γ-ray-burst sources, thought to be relatively old neutron stars, indicates that they are strongly magnetized and therefore that their fields have not decayed. One interpretation of the correlation observed3 between the strength of the magnetic field and the mass accreted by the neutron star is that mass accretion may itself lead to the decay of the magnetic field. Adopting the hypothesis of accretion-induced field decay, we calculate here the consequent evolution of a neutron star's spin and magnetic field. The results are consistent with observations of binary and millisecond radio pulsars. Thermomagnetic effects4 could provide a possible physical mechanism for such accretion-induced field decay.