Spin evolution of neutron stars in wind-fed high mass X-ray binaries.

The observed X-ray pulse period of OB-type high-mass X-ray binary (HMXB) pulsars are typically longer than 100 seconds. It is considered that the interaction between the strong magnetic field of neutron star and the wind matter could cause such a long pulse period. In this study, we follow the spin evolution of NS, taking into account the interaction between the magnetic field and wind matter. In this line, as new challenges, we solve the evolution of the magnetic field of the neutron star at the same time, and additionally we focus on the effects of wind properties of the donor. As the result, evolutionary tracks were obtained in which the neutron star spends some duration in the ejector phase after birth, then rapidly spins down, becomes quasi-equilibrium, and gradually spins up. Such evolution is similar to previous studies, but we found that its dominant physics depends on the velocity of the donor wind. When the wind velocity is fast, the spin-down occurs due to magnetic inhibition, while the classical propeller effect and settling accretion shell causes rapid spin-down in the slow wind accretion. Since the wind velocity of the donor could depend on the irradiated X-ray luminosity, the spin evolution track of the neutron star in wind-fed HMXB could be more complicated than considered.