Electromagnetic radiation from circular orbits in Schwarzschild--de Sitter spacetime

TL;DR

This paper studies electromagnetic radiation from charged particles orbiting Schwarzschild--de Sitter black holes, comparing it with scalar fields, and explores spectral features and power distributions near the photon sphere.

Contribution

It provides the first detailed analysis of electromagnetic emission in Schwarzschild--de Sitter spacetime, including power, spectral distribution, and comparison with scalar fields.

Findings

Electromagnetic power is about twice that of conformally coupled scalar fields at low angular velocities.

For minimal coupling, scalar power can surpass electromagnetic power.

Electromagnetic spectra are broader with significant low multipole contributions.

Abstract

We investigate the electromagnetic radiation emitted by a charged particle orbiting a four-dimensional Schwarzschild--de Sitter black hole using a semiclassical approach. We calculate the probability amplitude for the charged particle to emit a photon, from which we derive the emitted power and spectral distributions. The results are compared with those obtained for scalar fields, considering both minimal and nonminimal coupling, as well as the corresponding electromagnetic case in the Schwarzschild spacetime. It is found that the total electromagnetic power is approximately twice that of the scalar field with conformal coupling for orbits with low angular velocity, similar to the Schwarzschild case, whereas for minimal coupling, the scalar power can even exceed the electromagnetic power. We also investigate the spectral distributions and possible synchrotron emission by orbits near the photon sphere. The electromagnetic case has a much broader frequency spectrum, with a non-negligible lower multipole contribution.