Kinematics of Pulsar Beams

Abstract

We have investigated in detail the geometry of the open magnetic field line structure of an oblique dipole rotator, with a view to attaining a better understanding of the geometry of pulsar beams in the polar cap model of pulsar emission. We find that the open field lines divide into two branches, both of which are required to describe the full polar cap. We have also investigated the possible changes in pulsar beams due to the spacetime curvature caused by the neutron star and the special relativistic aberration. Barring the light bending, which is treated numerically, we incorporate all other effects analytically. The formalism can be used for an arbitrary emission altitude and for all inclination angles between the magnetic and rotation axes. The combination of all these effects surprisingly leaves the Goldreich-Julian type beam essentially unaltered, owing to the mutually opposing nature of these effects. The general relativistic effects at most give a 4% beam squeeze. At a finer level, the possibility of seeing the resultant small effects in pulsar observations is indicated and briefly discussed.