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

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 ori gin. We adopt the standard evolutionary model which implies that the neutro n star passes through several phases in a binary system ('isolated pulsar' - propeller - wind accretion - Roche lobe overflow). In the framework of th e model under consideration, the strong magnetic fields of relatively old p ulsars like Her X-1 and 4U 1626-67 can naturally be understood if, at their birth, they had a sufficiently strong magnetic field, similar to 3 x 10(13 ) G, comparable to the maximal field observed in radio pulsars.