Kerr black hole shadows from axion-photon coupling

Abstract

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 make constraints on the parameter space for such a theoretical model of the axion-photon coupling.