Dissecting Axions Around Supermassive Black Holes with Birefringence

With the upcoming project of the next-generation Event Horizon Telescope, the search for gravitational atoms made up of axions around supermassive black holes has become more and more promising. This new searching method makes use of the birefringence effect from the axion-photon coupling where a frequency-independent oscillation to the electric vector position angle of the linearly polarized radiation is predicted. The birefringent signals are influenced by the washout effects from the finite thickness of the accretion flow and the contribution from the lensed photons. In this talk, I will discuss the axion-induced birefringence for different cases of the black hole and accretion flow, including the impact of black hole inclination angles, spins and magnetic field, velocity distribution, the thickness of accretion flow, and finally I will give prospects on how future observations can remove these effects.