Axion Cloud Decay due to the Axion-photon Conversion with Background Magnetic Fields

Understand

We consider an axion cloud around a black hole with background magnetic fields.

• We calculate the decay rate of the axion cloud due to the axion-photon conversion associated with the axion-photon coupling.
• For simplicity, we consider the situation where the axion configuration is dominated by a solution for the eigenvalue equation equivalent to that for the Hydrogen atom, and the coupling term can be evaluated by a successive perturbation method.
• For the monopole background, we find the decay rate of the axion cloud is given by $\sim q^2\kappa^2(GM)^5\mu^8$, where $\mu$, $M$, $G$, $\kappa$ and $q$ are the axion mass, black hole mass, gravitational constant, coupling constant of the axion-photon coupling and monopole charge, respectively.