PSR J0045-7319: A dual-line binary radio pulsar

Binary radio pulsars are superb tools for mapping binary orbits because of the precision of the pulse timing method (e.g., Taylor & Weisberg). To date, all orbital parameters for binary pulsars have been derived from observations of the pulsar alone. We present the first observations of the radial velocity variations due to the binary motion of a companion to a radio pulsar. Our results demonstrate that the companion to the Small Magellanic Cloud pulsar J0045-7319 is the B1 V star identified by Kaspi et al. The mass ratio of the system is 6.3 ± 1.2, which, for a neutron star mass of 1.4M⊙, implies a mass of 8.8 ± 1.8 M⊙ for the companion, consistent with the mass expected for a B1 V star. The inclination angle for the binary system is therefore 44° ± 5°, and the projected rotational velocity of the companion is 113 ± 10 km s-1. The heliocentric radial velocity of the binary system is consistent with that of other stars and gas in the same region of the Small Magellanic Cloud.