A Photon Cloud Induced from an Axion Cloud

TL;DR

This paper demonstrates that an axion cloud around a Kerr black hole can induce an electromagnetic photon cloud through axion-photon coupling, which grows exponentially and could serve as an observable signature of axions.

Contribution

It provides the first classical analysis of EM photon cloud formation from an axion cloud in astrophysical environments, including analytical and numerical solutions.

Findings

Photon cloud oscillates at axion frequency

Photon cloud grows exponentially under superradiance

Induced EM field shows unique symmetry properties

Abstract

It is known that the axion-photon coupling can lead to quantum stimulated emission of photons and classic exponential amplification of electromagnetic (EM) fields at half the axion mass frequency, when the axion density or the coupling constant is sufficiently large. In this work, we studied the EM photon cloud induced from an axion cloud around a Kerr black hole in the first order of the coupling constant classically. In the presence of a static EM background (such as the extended Wald solution motivated by astrophysical environments), we found that an EM photon cloud emerges, oscillating at the same frequency as the axion cloud and growing exponentially in accordance with the axion cloud when the superradiant condition for the axion field is satisfied. The evolution of the EM photon cloud with time and azimuthal angle is obtained analytically while the cross-sectional distribution is solved numerically. The induced EM field exhibits symmetries that are markedly different from those of the background EM field. Consequently, the induced photon cloud forms an unstable bound configuration that emits EM waves to spatial infinity while being replenished by the axion cloud, providing a potential observational signature of both the presence of an axion cloud and axion-photon coupling.