The strong magnetic fields of the X-ray pulsars Hercules X-1 and 4U 1626–67 and their evolutionary scenarios

Abstract

We consider the magnetic and spin evolution of the X-ray binary pulsars Her X-1 and 4U 1626–67, assuming that their magnetic fields are of crustal origin. We adopt the standard evolutionary model which implies that the neutron star passes through several phases in a binary system (‘isolated pulsar’ — propeller — wind accretion — Roche lobe overflow). In the framework of the model under consideration, the strong magnetic fields of relatively old pulsars like Her X-1 and 4U 1626–67 can naturally be understood if, at their birth, they had a sufficiently strong magnetic field, ∼3 × 1013 G, comparable to the maximal field observed in radio pulsars.