Classical Spin-Orbit Coupling and Periastron Advance in PSR

. Radio timing observations of the SMC binary pulsar PSR J0045\Gamma7319 show, for the first time in a binary pulsar, that classical spin-orbit coupling and periastron advance are occurring. From the observations, the angle ` between the B star spin axis and the orbital angular momentum is constrained to be 25 ffi ! ` ! 41 ffi (Kaspi et al. 1996b). Under the conventional assumption that the pre-supernova angular momenta were aligned, this provides direct evidence for an asymmetric supernova explosion. In addition, a large, unexpected orbital decay is observed in the system. 1. Introduction PSR J0045\Gamma7319 is a 0.926-s radio pulsar in an eccentric 51-day binary in the SMC, as determined from pulse timing measurements at the Parkes telescope (Kaspi et. al. 1994). Its companion is a main-sequence B star, making this the second known pulsar/B star binary after PSR B1259\Gamma63 (see Johnston, this review). Doppler shifts of the companion's spectral lines have been detected at the ..