Black hole absorption cross sections: Spin and Regge poles

Abstract: We investigate the absorption of massless scalar, electromagnetic, and gravitational fields propagating in the Schwarzschild black hole geometry. Using complex angular momentum techniques, we first derive a representation of the absorption cross section that separates it into smooth background integrals and a discrete Regge pole series. This decomposition reveals the physical mechanisms underlying black hole absorption, including classical capture, surface wave interference near the photon sphere, and subleading background effects. We then construct a refined high-frequency analytical approximation that captures both the dominant oscillations and the fine structure of the absorption spectra for scalar, electromagnetic, and gravitational fields, incorporating spin-dependent phase corrections and higher-order effects. In addition, we provide a simplified expression that generalizes the sinc approximation to describe the leading oscillations for electromagnetic and gravitational fields. Our analysis offers a unified semiclassical interpretation of black hole absorption, combining geometric optics, surface wave dynamics, and resonant phenomena encoded by the Regge pole structure.