Scattering and absorption of gravitational plane waves by rotating black holes

This is a study of the scattering and absorption of planar gravitational waves by a Kerr black hole in vacuum. We apply the partial wave method to compute cross sections for the special case of radiation incident along the rotation axis. A catalogue of numerically-accurate cross sections is presented, for a range of incident wavelengths M ω ⩽ 4 and rotation rates a ⩽ 0.999 M . Three effects are studied in detail: polarization, helicity-reversal and glory scattering. First, a new approximation to the polarization in the long-wavelength limit is derived. We show that black hole rotation distinguishes between co-rotating and counter-rotating wave helicities, leading to a term in the cross section proportional to a ω. Second, we confirm that helicity is not conserved by the scattering process and show that superradiance amplifies the effect. For certain wavelengths, the back-scattered flux is enhanced by as much as ∼35 times for a rapidly-rotating hole (e.g. for a = 0.999 M at M ω = 0.945). Third, we observe regular glory and spiral scattering peaks in the numerically-determined cross sections. We show that the angular width and intensity of the peaks may be estimated via a semi-classical approximation. We conclude with a discussion of the observable implications of our results.