Absorption of electromagnetic and gravitational waves by Kerr black holes

We calculate the absorption cross section for planar waves incident upon Kerr black holes, and present a unified picture for scalar, electromagnetic and gravitational waves. We highlight the spin-helicity effect that arises from a coupling between the rotation of the black hole and the helicity of a circularly-polarized wave. For the case of on-axis incidence, we introduce an extended ‘sinc approximation’ to quantify the spin-helicity effect in the strong-field regime.