Evidence for Alignment of the Rotation and Velocity Vectors in Pulsars

Abstract

We review our case for strong observational evidence for a relationship between the direction of a pulsar's motion and its rotation axis. The information comes from calibrated polarization data for 20 pulsars which display linearly polarized emission from the pulse longitude at closest approach to the magnetic pole. Of these 20 pulsars, 10 show an offset in the angle between the velocity vector and the polarisation position angle which is either less than 10° or more than 80°, a fraction which is very unlikely by random chance. We believe that the bimodal nature of the distribution arises from the presence of orthogonal polarisation modes in the pulsar radio emission. In some cases this orthogonal ambiguity is resolved by observations at other wavelengths so that we conclude that the velocity vector and the rotation axis are aligned at birth. Strengthening the case is the fact that 4 of the 5 pulsars with ages less than 3 Myr show this relationship, including the Vela pulsar.