A model of gamma ray pulsars: Emission from single polar caps on nearly aligned rotators

We interpret CGRO observations of gamma ray pulsars in terms of the polar cap cascade model of gamma ray emission. Our results are based on a revised simulation of cascade showers induced by curvature radiation in pulsar magnetospheres. We find that models in which the magnetic and rotational axes are nearly aligned, and in which the primary electron beam is concentrated around the rim of the polar cap, can reproduce basic features of the observed pulse profiles and spectra. The beaming factors expected from nearly aligned rotators imply that their luminosities need not be as large as for orthogonal rotators. However, small beam angles are also a difficulty with this model since they imply low detection probabilities. We note that the usual estimates of polar cap radii assume pure dipole geometries, and that the estimated values should be increased due to plasma loading of the field lines.