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 omega <= 4 and rotation rates a <= 0.999M. 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.. 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 similar to 35 times for a rapidly-rotating hole (e.g. for a = 0.999M at M omega = 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.