Series reduction method for scattering of planar waves by Kerr-Newman black holes

We present a practical method for evaluating the scattering amplitude $f_s(θ,ϕ)$ that arises in the context of the scattering of scalar, electromagnetic and gravitational planar waves by a rotating black hole. The partial-wave representation of $f_s$ is a divergent series, but $f_s$ itself diverges only at a single point on the sphere. Here we show that $f_s$ can be expressed as the product of a reduced series and a pre-factor that diverges only at this point. The coefficients of the reduced series are found iteratively as linear combinations of those in the original series, and the reduced series is shown to have amenable convergence properties. This series-reduction method has its origins in an approach originally used in electron scattering calculations in the 1950s, which we have extended to the axisymmetric context for all bosonic fields.

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