Extraction of energy from an extremal rotating electrovacuum black hole: Particle collisions in the equatorial plane

TL;DR

This paper investigates energy extraction via particle collisions in extremal rotating electrovacuum black holes, showing that unbounded energy extraction is possible with small black hole charges, offering insights into high-energy astrophysical processes.

Contribution

It provides a unified analysis of the collisional Penrose process for charged particles in rotating black holes, revealing conditions under which energy bounds can be exceeded.

Findings

Unbounded energy extraction possible with small black hole charges.

Energy bounds are absent for arbitrarily small electric charges.

The model is relevant for high-energy processes around astrophysical black holes.

Abstract

The collisional Penrose process received much attention when Banados, Silk and West (BSW) pointed out the possibility of test-particle collisions with arbitrarily high center-of-mass energy in the vicinity of the horizon of an extremally rotating black hole. However, the energy that can be extracted from the black hole in this promising, if simplified, scenario, called the BSW effect, turned out to be subject to unconditional upper bounds. And although such bounds were not found for the electrostatic variant of the process, this version is also astrophysically unfeasible, since it requires a maximally charged black hole. In order to deal with these deficiencies, we revisit the unified version of the BSW effect concerning collisions of charged particles in the equatorial plane of a rotating electrovacuum black hole spacetime. Performing a general analysis of energy extraction through this process, we explain in detail how the seemingly incompatible limiting cases arise. Furthermore, we demonstrate that the unconditional upper bounds on the extracted energy are absent for arbitrarily small values of the black hole electric charge. Therefore, our setup represents an intriguing simplified model for possible highly energetic processes happening around astrophysical black holes, which may spin fast but can have only a tiny electric charge induced via interaction with an external magnetic field.