# A new type of charged black hole bomb

## TL;DR

This paper introduces a novel black hole bomb configuration where the mirror is outside the field, still causing instabilities if inside the ergoregion, demonstrated through numerical simulations on charged black hole backgrounds.

## Contribution

It presents a new black hole bomb setup with the mirror outside the field, expanding understanding of superradiant instabilities in charged black holes.

## Key findings

- Instabilities occur when the mirror is inside the ergoregion.
- Numerical simulations confirm the new configuration causes linear instabilities.
- The method applies to Reissner-Nordstrom and de Sitter-Reissner-Nordstrom backgrounds.

## Abstract

Black hole bombs are usually constructed by surrounding an ergoregion by a mirror. The fields propagating between the event horizon and the mirror are prevented from escaping to infinity and reflected back to the ergoregion, thus undergoing repeated superradiant scattering which leads to a linear instability. We introduce a new construction in which the field is outside the mirror and is therefore prevented from falling into the black hole but is free to escape to infinity. Provided the mirror is inside the ergoregion, it turns out that this still causes linear instabilities. This behaviour is observed on Reissner-Nordstrom and de Sitter-Reissner-Nordstrom backgrounds using numerical simulations, based on a semi-implicit discretisation on a first-order system formulation of the partial differential equations governing the evolution of the scalar field. We also perform simulations for a standard black hole bomb and for another type of contraption: a sandwich black hole bomb.