With high spatial resolution, polarimetric imaging of a supermassive black hole, like M87^\star or Sgr A^\star, by the Event Horizon Telescope can be used to probe the existence of ultralight bosonic particles, such as axions. Such particles can accumulate around a rotating black hole through superradiance mechanism, forming an axion cloud. When linearly polarized photons are emitted from accretion disk near the horizon, their position angles oscillate due to the birefringent effect when traveling through the axion background.

In particular, the supermassive black hole M87^\star (Sgr A^\star) can probe axions with masses O(10^{-20}) eV (O( 10^{-17}) eV) and decay constant smaller than O(10^{16}) GeV, which is complimentary to black hole spin measurements.