Resonant Photon-Axion Mixing Driven by Dark Matter Oscillations

TL;DR

This paper demonstrates that coherent axion dark matter backgrounds can induce resonant photon-axion conversion through driven mode mixing, revealing new polarization effects and implications for astrophysical observations.

Contribution

It introduces a Floquet theory-based framework for understanding resonant photon-axion mixing driven by dark matter oscillations, highlighting phenomena missed by standard static models.

Findings

Resonant conversion occurs at specific frequency mismatches related to axion oscillation harmonics.

The process preserves axion dark matter density and is distinct from decay or parametric instabilities.

Characteristic polarization signatures could be observed in astrophysical data.

Abstract

Wave propagation in periodically time-dependent media can exhibit driven mode conversion that is absent in static or adiabatic descriptions. We show that photon propagation through a coherent axion dark matter background provides a natural realization of such driven dynamics. In the presence of a magnetic field, the oscillating axion field acts as a coherent temporal drive, inducing resonant photon-axion conversion when the mismatch between their dispersion relations is compensated by integer harmonics of the axion oscillation frequency, $\Delta_\gamma - \Delta_a \approx n m_a$ with $n \in \mathbb{Z}$. This driven resonance enables efficient mixing far from the conventional level-crossing regime and disappears entirely upon time averaging, explaining why it is missed in standard treatments. The process constitutes a unitary mode-conversion phenomenon that preserves the axion dark matter number density and is distinct from parametric instabilities or axion decay. A systematic description is naturally provided by Floquet theory. We develop a general framework for photon propagation in oscillating axion backgrounds and show that the resulting resonant mixing leads to characteristic polarization signatures, with potential implications for astrophysical observations such as blazar polarization.