Kerr black hole shadows from the axion-photon coupling

TL;DR

This paper studies how axion-photon coupling affects the shadows of Kerr black holes, revealing slight size increases and unique structures, with implications for real black hole observations.

Contribution

It introduces the effects of axion-photon coupling on Kerr black hole shadows, including size changes and novel features, under weak coupling approximation.

Findings

Shadow size slightly increases with coupling.

Pedicel-like structures appear in rapid rotation cases.

Model remains consistent with observed black hole shadows.

Abstract

We have investigated the motion for photons in the Kerr black hole spacetime under the axion-photon coupling. The birefringence phenomena arising from the axion-photon coupling can be negligible in the weak coupling approximation because the leading-order contributions to the equations of motion come from the square term of the coupling parameter. We find that the coupling parameter makes the size of shadows slightly increase for arbitrary spin parameter. For the rapid rotating black hole case with a larger coupling, we find that there exist a pedicel-like structure appeared in the left of the D-type like shadows.Comparing the shadow size of the Kerr black hole with the shadow size of the Sgr A* and M87* black holes, we find that there is room for such a theoretical model of the axion-photon coupling.