Stringent axion constraints with Event Horizon Telescope polarimetric measurements of M87$^\star$

Understand

The hitherto unprecedented angular resolution of the Event Horizon Telescope (EHT) has created exciting opportunities in the search for new physics.

• Recently, the linear polarization of radiation emitted near the supermassive black hole M87$^\star$ was measured on four separate days, precisely enabling tests of the existence of a dense axion cloud produced by a spinning black hole.
• The presence of an axion cloud leads to a frequency-independent oscillation in the electric vector position angle (EVPA) of this linear polarization.
• For a nearly face-on M87$^\star$, this oscillation in the EVPA appears as a propagating wave along the photon ring.