Lightlike boost of the Kerr gravitational field

We describe lightlike 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 lightlike boosts of the Kerr gravitational field. Our conclusions confirm the correctness of the limits calculated by Balasin and Nachbagauer [Class. Quantum Grav. 13, 731 (1996)]. To avoid any ambiguity our approach is centered on evaluating the lightlike boost of the Riemann tensor for the Kerr space-time with the metric playing a secondary role.