Collimation effects of the Kerr field

Abstract

The latitudinal motion of free test particles outwards from central regions in the Kerr geometry is investigated analytically and by the numerical integration of geodesic equations. Compared to particles with parabolic energies, particles with hyperbolic energies escape closer to the rotation axis, while slower particles tend to fall towards the equatorial plane. The effect is not large in the case of Kerr black holes, but it is significant for Kerr naked singularities. In the region even closer to the centre, the repulsive character of the field near the rotation axis and near the disc spanning the Kerr ring singularity gives particles outward accelerations and collimates them along the rotation axis. Here, low-energy particles are more collimated than are those with high energies. These results might be relevant in the context of the formation and precollimation of cosmic jets