Resonant photon-axion conversion driven by axion dark matter oscillations

Abstract

Wave propagation in periodically time-dependent media can exhibit driven mode conversion that is absent in static or adiabatic descriptions. We show that coherent axion dark matter provides an intrinsic realization of this dynamics for photons propagating in a magnetic field. The background's periodic birefringence continuously rotates the photon polarization and thereby drives magnetic photon--axion mixing. Using Floquet theory, we derive resonant photon--axion conversion when integer harmonics of the axion oscillation frequency compensate the dispersion mismatch, ${\Delta }_{\gamma }-{\Delta }_a\approx n{m}_a$ with $n\in \mathbb{Z}$. This resonance enables efficient mixing far from conventional level crossing and is lost when the periodic drive is averaged out of the propagation Hamiltonian. The resulting conversion is bounded and unitary, distinct from parametric instability or axion decay. Our framework describes photon propagation in coherent oscillating axion backgrounds and predicts characteristic polarization signatures, which we illustrate in a blazar environment.

7+10 pages, 5 figures