SPIN-ORBIT INTERACTION IN NEUTRON STAR/MAIN SEQUENCE BINARIES AND IMPLICATIONS FOR PULSAR TIMING

The spin-induced quadrupole moment of a rapidly rotating star changes the orbital dynamics in a binary system, giving rise to advance (or regression) of periastron and precession of the orbital plane. We show that these effects are important in the recently discovered radio pulsar/main sequence star binary system PSR J0045$-$7319, and can reliably account for the observed peculiar timing residuals. Precise measurements of the apsidal motion and orbital plane precession can yield valuable information on the internal structure and rotation of the star. The detection of orbital precession implies that the spin of the companion star is not aligned with the orbital angular momentum, and suggests that the supernova gave the pulsar a kick out of the original orbital plane. Tidal excitation of g-mode oscillations in the PSR J0045$-$7319 system induces an orbital period change of order $|\Delta P_{\rm orb}/P_{\rm orb}|\sim 10^{-6}$ at each periastron passage, but the secular trend depends on the radiative damping time of the g-modes. We also discuss the spin-orbit coupling effects for the accreting X-ray pulsars and the other known radio pulsar/main sequence binary, PSR B1259$-$63.