Signatures of particle collisions near extreme black holes

TL;DR

This paper investigates the observable signatures of high-energy particle collisions near extreme Kerr black holes, providing a universal framework to analyze emission properties and applying it to photon bremsstrahlung.

Contribution

It introduces a universal approach to determine emission signatures from near-horizon collisions in extreme Kerr geometry, applicable to various scattering processes.

Findings

Ejected particles are observable but energy is bounded by the rest masses of colliding particles.

The framework simplifies the calculation of emission properties from near-horizon collisions.

Application to photon bremsstrahlung confirms previous results about observability.

Abstract

Finite-energy particles in free fall can collide with diverging center-of-mass energy near rapidly rotating black holes. What are the most salient observational signatures of this remarkable geometric effect? Here we revisit the problem from the standpoint of the near-horizon extreme Kerr geometry, where these collisions naturally take place. It is shown that the ingoing particle kinematics admits a simple, universal form. Given a scattering cross section, determination of emission properties is reduced to evaluation of particular integrals on the sky of a near-horizon orbiting particle. We subsequently apply this scheme to the example of single-photon bremsstrahlung, substantiating past results which indicate that ejected particles are observable, but their energies are bounded by the rest masses of the colliding particles. Our framework is readily applicable for any scattering process.