Birefringence Tomography for Axion Cloud

Understand

An axion cloud surrounding a supermassive black hole can be naturally produced through the superradiance process.

• Its existence can be examined by the axion induced birefringence effect.
• It predicts an oscillation of the electric vector position angle of linearly polarized radiations.
• Stringent constraints of the existence of the axion in a particular mass window has been obtained based on the recent Event Horizon Telescope measurement on M87$^\star$.