Non-minimal coupling of photons and axions

TL;DR

This paper develops a new theoretical framework for non-minimal interactions between gravitational, electromagnetic, and axion fields, exploring exact solutions and empirical constraints relevant to optical activity and polarization in astrophysics.

Contribution

It introduces a self-consistent set of equations for non-minimal photon-axion coupling extending the Einstein-Maxwell-axion theory with a ten-parameter model.

Findings

Derived exact solutions for electromagnetic wave propagation with non-minimal coupling.

Identified potential optical activity effects induced by axions.

Discussed observational constraints from astrophysical birefringence and polarization data.

Abstract

We establish a new self-consistent system of equations accounting for a non-minimal interaction of gravitational, electromagnetic and axion fields. The procedure is based on a non-minimal extension of the standard Einstein-Maxwell-axion action. The general properties of a ten-parameter family of non-minimal linear models are discussed. We apply this theory to the models with pp-wave symmetry and consider propagation of electromagnetic waves non-minimally coupled to the gravitational and axion fields. We focus on exact solutions of electrodynamic equations, which describe quasi-minimal and non-minimal optical activity induced by the axion field. We also discuss empirical constraints on coupling parameters from astrophysical birefringence and polarization rotation observations.