A Ferromagnetic Model of Neutron Stars and the Spin Evolution of Anomalous X-Ray Pulsars

Abstract

The spin evolution of neutron stars possessing a ferromagnetic core is investigated by focusing on the electromagnetic coupling of the core and the crust. Differential rotations of the core and the crust are shown to produce a huge energy dissipation in the crust owing to induction currents, thereby enhancing the stellar luminosity by two to three orders of magnitude beyond the dipole luminosity. Identifying the pulse period of a pulsar with the spin period of the core, the evolution of the pulse period involves a stationary era that is similar to that expected when a magnetic field decays. This ferromagnetic core model (FCM) is applied to the anomalous X-ray pulsars 1E 1841-045 and 1E 2259+586 to resolve some problems concerning their ages and luminosities. The importance of the braking index is noted to discriminate the FCM from the field-decay model.