Detecting Axion Dark Matter with Black Hole Polarimetry

The axion, as a leading dark matter candidate, is the target of many ongoing and proposed experimental searches based on its coupling to photons. Ultralight axions that couple to photons can also cause polarization rotation of light, which the cosmic microwave background can probe. In this work, we show that a large axion field is inevitably developed around supermassive black holes due to the Bose-Einstein condensation of axions, enhancing the induced birefringence effects. Therefore, measuring the modulations of supermassive black hole imaging polarization angles is a strong probe of the axion-photon coupling due to the formation of the axion condensation (axion star) which enhances the axion field. The oscillating axion field around black holes induces polarization rotation on the black hole image, which is detectable and distinguishable from astrophysical effects on the polarization angle, as it exhibits distinctive temporal variability and frequency invariability. In this work, we perform theoretical calculations of the axion star formation rate and the corresponding enhanced axion field value near supermassive black holes. Then, we present the range of axion-photon couplings within the axion mass range $10^{-21}-10^{-16}\,\text{eV}$ that can be probed by the Event Horizon Telescope. The axion parameter space probed by black hole polarimetry will expand with improvements in sensitivity of polarization measurements and more black hole polarimetry targets with determined black hole masses.