Stability of Axion Dark Matter-Photon Conversion

TL;DR

This paper investigates how the presence of cosmological magnetic fields affects the stability and resonance conditions of axion-photon conversion, revealing shifted bifurcation points and new instability bands due to the coupled system dynamics.

Contribution

It demonstrates that magnetic fields alter the instability conditions of axion-photon systems, introducing new resonance behaviors and stability criteria compared to the standard Mathieu equation.

Findings

Magnetic fields shift bifurcation points in the axion-photon system.

New instability bands emerge due to axion-photon coupling.

Resonant axion-photon conversion is affected by magnetic field presence.

Abstract

It is known that a coherently oscillating axion field is a candidate of the dark matter. In the presence of the oscillating axion, the photon can be resonantly produced through the parametric amplification. In the universe, there also exist cosmological magnetic fields which are coherent electromagnetic fields. In the presence of magnetic fields, an axion can be converted into a photon, and vice versa. Thus, it is interesting to investigate what happens for the axion-photon system in the presence of both the axion dark matter and the magnetic fields. This system can be regarded as a coupled system of the axion and the photon whose equations contain the Mathieu type terms. We find that the instability condition is changed in the presence of magnetic fields in contrast to the conventional Mathieu equation. The positions of bifurcation points between stable and unstable are shifted and new instability bands appear. This is because the resonantly amplified axion can be converted to photon, and vice versa.