Circular polarization effects induced by photon-axion mixing in astrophysical environments

Abstract

Axions and axion-like particles (ALPs) are compelling candidates for dark matter and new physics beyond the Standard Model. Photon-axion mixing in external magnetic fields modifies the photon energy spectrum and linear polarization state, and also induces circular polarization signals. Compared to spectral and linear polarization methods, circular polarization benefits from lower astrophysical background contamination, providing an independent probe for axion searches. In this work, we study the circular polarization induced by photon-axion mixing within the chiral basis framework. By analytically solving the evolution equations under the single-domain approximation, we derive an expression for the circular polarization degree $P_C$, applicable in the resonant, strong coupling, and weak coupling regimes. Within single-domain magnetic field models, we compare the energy-dependent circular polarization in four astrophysical environments (AGN jets, intracluster medium, intergalactic medium, and Galactic magnetic fields). We find that the X-ray to MeV band represents the most sensitive observational window. Using the blazar S4 0954+65 as a case study, phase accumulation in random magnetic domains causes the circular polarization degree to fluctuate with redshift and exhibit pronounced energy structures. Using the optical circular polarization upper limit $P_C < 0.184\%$ (measured in the z-SDSS band) from this source, we statistically constrain $g_{a\gamma\gamma} \lesssim 3\times 10^{-11}\,\mathrm{GeV}^{-1}$ (95\% confidence level) for $m_a \sim 10^{-16}$--$10^{-10}\ \mathrm{eV}$, with the strongest constraint reaching $g_{a\gamma\gamma}\sim6\times10^{-12}\,\mathrm{GeV}^{-1}$ near $m_a \sim 10^{-14}\,\mathrm{eV}$. These results establish circular polarization as a complementary axion probe.