Geometry of pulsar beams - Relative orientations of rotation axis, magnetic axis, and line of sight

Abstract

The observed polarisation angle variation in pulsars is analysed on the basis of the magnetic pole model. The angles alpha between the rotation and magnetic axes and beta between magnetic axis and line of sight are estimated using (1) the shape of the polarisation angle (theta) vs pulse longitude (phi) curve, (2) the magnitude of the central gradient d theta/d phi in the main pulse, and (3) the magnitude and sign of the gradient in the interpulse. It is concluded that beta can have either sign, with outer lines of sight (i.e., away from the rotation axis) probably predominating. This result is contrary to certain theories which predict a complete absence of outer lines of sight. It is also concluded that alpha is not randomly distributed; small angles seem to be preferred.