Relativistic Spin Precession in the Binary PSR J1141-6545

PSR J1141-6545 is a precessing binary pulsar that has the rare potential to reveal the two-dimensional structure of a non-recycled pulsar emission cone. It has undergone similar to 25 degrees of relativistic spin precession in the similar to 18 yr since its discovery. In this Letter, we present a detailed Bayesian analysis of the precessional evolution of the width of the total intensity profile, in order to understand the changes to the line-of-sight (LOS) impact angle (beta) of the pulsar using four different physically motivated prior distribution models. Although we cannot statistically differentiate between the models with confidence, the temporal evolution of the linear and circular polarizations strongly argue that our LOS crossed the magnetic pole around MJD 54,000 and that only two models remain viable. For both of these models, it appears likely that the pulsar will precess out of our LOS in the next 3-5 yr, assuming a simple beam geometry. Marginalizing over beta suggests that the pulsar is a near-orthogonal rotator and provides the first polarization-independent estimate of the scale factor (A) that relates the pulsar beam opening angle (rho) to its rotational period (P) as rho = AP(-0.5): we find it to be >6 degrees s(0.5) at 1.4 GHz with 99% confidence. If all pulsars emit from opposite poles of a dipolar magnetic field with comparable brightness, we might expect to see evidence of an interpulse arising in PSR J1141-6545, unless the emission is patchy.