Absorption and unbounded superradiance in a static regular black hole spacetime

Regular black holes (RBHs)—geometries free from curvature singularities—arise naturally in theories of nonlinear electrodynamics. Here we study the absorption and superradiant amplification of a monochromatic planar wave in a charged, massive scalar field impinging on the electrically charged Ayón-Beato-García (ABG) RBH. Comparisons are drawn with absorption and superradiance for the Reissner-Nordström (RN) black hole in linear electrodynamics. We find that, in a certain parameter regime, the ABG absorption cross section is negative, due to superradiance, and moreover it is unbounded from below as the momentum of the wave approaches zero; this phenomenon of “unbounded superradiance” is absent in the RN case. We show how the parameter space can be divided into regions, using the bounded/unbounded and absorption/amplification boundaries. After introducing a high-frequency approximation based on particle trajectories, we calculate the absorption cross section numerically, via the partial-wave expansion, as a function of wave frequency, and we present a gallery of results. The cross section of the ABG RBH is found to be larger (smaller) than in the RN case when the field charge has the same (opposite) sign as the black hole charge. We show that it is possible to find “mimics”: situations in which the cross sections of both black holes are very similar. We conclude with a discussion of unbounded superradiance and superradiant instabilities.