Orbital Decay of the PSR J0045-7319 Binary System: Age of Radio Pulsar and Initial Spin of Neutron Star

Recent timing observations of PSR J0045-7319 reveal that the neutron star/B star binary orbit is decaying on a time scale of |Porb/Ṗorb| = 0.5 Myr, shorter than the characteristic age (τc = 3 Myr) of the pulsar (Kaspi et al. 1996a). We study mechanisms for the orbital decay. The standard weak friction theory based on static tide requires far too short a viscous time to explain the observed Ṗorb. We show that dynamical tidal excitation of g-modes in the B star can be responsible for the orbital decay. However, to explain the observed short decay timescale, the B star must have some significant retrograde rotation with respect to the orbit — The retrograde rotation brings lower-order g-modes, which couple much more strongly to the tidal potential, into closer “resonances” with the orbital motion, thus significantly enhancing the dynamical tide. A much less likely possibility is that the g-mode damping time is much shorter than the ordinary radiative damping time. The observed orbital decay timescale combined with a generic orbital evolution model based on dynamical tide can be used as a “timer”, giving an upper limit of 1.4 Myr for the age of the binary system since the neutron star formation. Thus the characteristic age of the pulsar is not a good age indicator. Assuming standard magnetic dipole braking for the pulsar and no significant magnetic field decay on a timescale <∼ 1 Myr, the upper limit for the age implies that the initial spin of the neutron star at birth was close to its current value. Subject headings: binaries: close – pulsars: individual (PSR J0045-7319) – stars: neutron – stars: oscillations – stars: rotation – hydrodynamics – supernovae