Effect of a Weak Electromagnetic Field on Particle Acceleration by a Rotating Black Hole

TL;DR

This paper investigates how electromagnetic fields influence particle collisions near rotating black holes, revealing conditions for arbitrarily high energy outcomes and highlighting the role of black hole spin and magnetic fields in astrophysical acceleration processes.

Contribution

It demonstrates that magnetic fields can enhance particle acceleration near rotating black holes, but the process's stability depends on black hole spin, extending previous models to more realistic astrophysical scenarios.

Findings

Arbitrarily high CM energy is achievable near maximally rotating black holes with magnetic fields.

High-energy particle orbits near black holes require infinite energy in strong field limits.

Magnetic fields can significantly boost black holes' potential as particle accelerators.

Abstract

We study high energy charged particle collisions near the horizon in an electromagnetic field around a rotating black hole and reveal the condition of the fine-tuning to obtain arbitrarily large center-of-mass (CM) energy. We demonstrate that the CM energy can be arbitrarily large as the uniformly magnetized rotating black hole arbitrarily approaches maximal rotation under the situation that a charged particle plunges from the innermost stable circular orbit (ISCO) and collides with another particle near the horizon. Recently, Frolov [Phys. Rev. D 85, 024020 (2012)] proposed that the CM energy can be arbitrarily high if the magnetic field is arbitrarily strong, when a particle collides with a charged particle orbiting the ISCO with finite energy near the horizon of a uniformly magnetized Schwarzschild black hole. We show that the charged particle orbiting the ISCO around a spinning black hole needs arbitrarily high energy in the strong field limit. This suggests that Frolov's process is unstable against the black hole spin. Nevertheless, we see that magnetic fields may substantially promote the capability of rotating black holes as particle accelerators in astrophysical situations.