Event Details

With high spatial resolution, polarimetric imaging of supermassive black hole, like M87* or Sgr A*, 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. Recently, the polarization properties of the radiation near the supermassive black hole M87* are measured in four individual days. This is exactly what is needed to test the existence of a dense axion cloud. We apply the azimuthal distribution of EVPA measured by the EHT and study the axion-photon coupling.  The EHT data can rule out a considerable portion of the axion parameter space for the axion mass window from 10^−21 to 10^−20 eV, which was unexplored by previous experiments.