Constraints on Axion-like Dark Matter from Cosmic Birefringence with a Polarization Array of Repeating Fast Radio Bursts

Abstract

Axion-like dark matter (ALDM) is a well-motivated dark matter candidate whose Chern-Simons coupling to photons can induce cosmic birefringence, producing temporal oscillations and spatial correlation in polarization angle (PA) signals from different sources. Hyperactive repeating fast radio bursts (FRBs) with high linear polarization are a particularly promising probe of such signatures. We develop a Bayesian framework to search for PA oscillations caused by ALDM in individual repeating FRBs, and combine multiple sources in an FRB polarization array (FRBPA) by exploiting correlations arising from the common Earth term. We apply this framework to PA datasets from three well-studied hyperactive repeating FRBs, and constrain the Chern-Simons coupling over a broad range of ALDM masses. For each FRB, we estimate the dark matter density at its host location using a simulation-calibrated halo profile. The inferred dark matter densities are comparable to the fiducial local density $ρ_0 = 0.4~\mathrm{GeV/cm^3}$. The comparable source and Earth dark matter densities, together with the redshift-induced difference between the frequencies of the source and Earth terms, motivate our adoption of the full two-component birefringence model. We find no compelling evidence for ALDM-induced PA oscillations in either the individual source analyses or the joint FRBPA analysis. We place 95\% credible upper limits on the Chern-Simons coupling strength as a function of ALDM mass. Across the axion mass from $10^{-23}$ to $10^{-18}~\mathrm{eV}$, the resulting upper limits of the coupling strength of FRBPA lie between $10^{-12.6}$ and $10^{-7.9}~\mathrm{GeV}^{-1}$. Our results establish long-term polarimetric monitoring of hyperactive repeating FRBs as a promising and independent probe of ALDM, with sensitivity expected to improve with longer observational baselines and larger source samples.