Kerr geodesics, the Penrose process and jet collimation by a black hole

Aims. We re-examine the possibility that astrophysical jet collimation may arise from the geometry of rotating black holes and the presence of high-energy particles resulting from a Penrose process, without the help of magnetic fields. Methods. Our analysis uses the Weyl coordinates, which are revealed better adapted to the desired shape of the jets. We numerically integrate the 2D-geodesics equations. Results. We give a detailed study of these geodesics and give several numerical examples. Among them are a set of perfectly collimated geodesics with asymptotes rho = rho(1) parallel to the z-axis, with rho(1) only depending on the ratios Q/E-2-1 and a/M, where a and M are the parameters of the Kerr black hole, E the particle energy and Q the Carter's constant.