Gravitational shockwaves on rotating black holes

We present an exact solution of Einstein's equation that describes the gravitational shockwave of a massless particle on the horizon of a Kerr–Newman black hole. The backreacted metric is of the generalized Kerr–Schild form and is Type II in the Petrov classification. We show that if the background frame is aligned with shear-free null geodesics, and if the background Ricci tensor satisfies a simple condition, then all nonlinearities in the perturbation will drop out of the curvature scalars. We make heavy use of the method of spin coefficients (the Newman–Penrose formalism) in its compacted form (the Geroch–Held–Penrose formalism).