Neutron star evolution in wide low-mass X-ray binaries

The low magnetic field strengths of binary and millisecond radio pulsars are generally suggested to be related to their prior mass transfer phase in binary evolution. There is evidence for magnetic field decay in neutron stars that have experienced accretion in a binary system. To examine the accretion-induced field decay hypothesis, we calculate the evolutionary history of a magnetized neutron star in a wide low-mass X-ray binary (LMXB) driven by the nuclear evolution of the evolved companion. These binary systems are thought to be the progenitors of wide binary radio pulsars, and they form a homogeneous group whose evolutionary history can be solved semi-analytically. In our calculation, nonconservative evolution during mass transfer is assumed, and a possible formula for field decay with mass accretion is proposed. We use several accretion torque models to calculate the spin history of the neutron star. The predicted spin periods, orbital periods and magnetic field strengths of the neutron star are in accord with the observations of eight wide binary radio pulsars. Our results support accretion-induced field decay as a plausible scenario for the evolution of the neutron star magnetic field and present constraints on the possible fields decay forms with mass accretion and on the accretion torque models.