Lightlike boost of the Kerr gravitational field

Abstract

We describe light-like boosts of the Kerr gravitational field transverse and parallel to the symmetry axis. In the transverse case the boosted field is that of an impulsive gravitational wave having a line singularity displaced relative to its position if the rotation of the source were removed. The parallel boost is insensitive to the rotation of the source. The literature contains a number of diverse results for light-like boosts of the Kerr gravitational field. Our conclusions confirm the correctness of the limits calculated by Balasin and Nachbagauer [Class.and Quantum Grav.13(1996),731]. To avoid any ambiguity our approach is centered on evaluating the light-like boost of the Riemann tensor for the Kerr space-time with the metric playing a secondary role.