Porb RELATION FOR WIND-FED X-RAY PULSARS

We have investigated the relation between the orbital periods (Porb) and the spin periods (Ps) of wind-fed X-ray pulsars in high-mass X-ray binaries (HMXBs), based on population synthesis calculations of the spin evolution of neutron stars during the pre-HMXB stage. We show that most of the neutron stars either have steady accretion or still reside in the radio pulsar phase when the donor star starts evolving off the main sequence. In the former case the values of Ps can be decelerated to be ∼ 10 − 10 s depending on Porb. We briefly discuss the possible origin of the Ps−Porb correlation in Be/X-ray binaries, and the existence of HMXBs with main sequence donors. We have also investigated the evolution of the inclination angle between the magnetic and spin axes of neutron stars in a massive binary, suggesting secular alignment of the magnetic and spin axes during their evolution. Subject headings: stars: early-type — X-rays: binaries — stars: evolution — stars: neutron