2006ApJ...653.1410D

We have investigated the relation between the orbital periods (P_orb) and the spin periods (P_s) 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 neutron stars either have steady accretion or still reside in the radio pulsar phase when the donor star starts to evolve off the main sequence. In the former case, the spin period can be decelerated to ∼10²-10³ s, depending on the value of P_orb. We briefly discuss possible origins of the P_s-P_orb correlation in Be/X-ray binaries and the existence of HMXBs with main-sequence donors. We also investigate 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.