  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 the
superradiance mechanism, forming an axion cloud. When linearly polarized
photons are emitted from an accretion disk near the horizon, their position
angles oscillate due to the birefringent effect when traveling through the
axion background. In particular, the observations of supermassive black holes
M87$^\star$ (Sgr A$^\star$) can probe the dimensionless axion-photon coupling
$c = 2 \pi g_{a \gamma} f_a$ for axions with mass around $O(10^{-20})$~eV ($O(
10^{-17}$)~eV) and decay constant $f_a < O(10^{16})$ GeV, which is
complimentary to other axion measurements.
