A model for period clustering of long-period pulsars

We explore the possibility that the P -(P overdot) distribution of observed pulsars can be understood in terms of a model of a rotating neutron star that possesses two types of magnetic fields, a fossil magnetic field of O(10(10-13)) G at the stellar surface, and the field due to the ferromagnetic core of a neutron star, which is assumed to be of O(10(14-15)) G. This model, when combined with the assumption of an appropriate mass distribution, together with the observational selection effect on an assumed magnetic dipole pulse luminosity proportional to P-4. is shown to be consistent with the 'period clustering' of the observed SGRs and AXPs.