Speaker
Description
The axion or axion like particle (ALP), as a leading dark matter candidate, is the target of many on-going and proposed experimental searches based on its coupling to photons. However, indirect searches for axions have not been as competitive as direct searches that can probe a large range of parameter space. In this talk, I will introduce the idea that axion stars will inevitably form in the vicinity of supermassive black holes due to Bose-Einstein condensation, enhancing the axion birefringence effect and opening up more windows for axion indirect searches. 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. We show that the polarization measurement from Event Horizon Telescope can set the most competitive limit on axions in the mass range of $10^{-21}$-$10^{-16}$ eV.
