Near-extremal quantum cross-section for charged fields and superradiance

TL;DR

This paper analyzes quantum effects on charged scalar fields scattering off near-extremal Reissner-Nordström black holes, revealing enhanced superradiance and increased absorption cross-section due to quantum fluctuations described by the Schwarzian action.

Contribution

It demonstrates the impact of quantum fluctuations on scattering and absorption properties in near-extremal black holes, highlighting the role of the Schwarzian effective action in this regime.

Findings

Enhanced reflection coefficient in superradiant regime

Increased absorption cross-section in both regimes

Kinks indicating shutdown of absorption and stimulated emission

Abstract

We study the scattering and absorption properties of charged scalar fields on a near-extremal Reissner-Nordstr\"om black hole background. We show that in this low-temperature regime the near-horizon throat experiences large quantum fluctuations, whose leading contribution is described by the one dimensional Schwarzian effective action, while the soft $U(1)$ gauge modes can only contribute to subleading order. We investigate the role of the leading quantum effects both inside and outside the superradiant regime. These effects result in an enhanced reflection coefficient within the superradiant regime, while causing a suppression in the non-superradiant regime. On the other hand, the absorption cross-section increases in both regimes. Additional physical features appear as kinks in the reflection coefficient and absorption cross-section plots, corresponding to the shutdown of absorption in the superradiant regime and of stimulated emission in the non-superradiant regime.