Stringent axion constraints with Event Horizon Telescope polarimetric measurements of M87

The hitherto unprecedented angular resolution of the Event Horizon Telescope has created exciting opportunities in the search for new physics. Recently, the linear polarization of radiation emitted near the supermassive black hole M87* 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 of this linear polarization. For the nearly face-on M87*, this oscillation in the electric vector position angle appears as a propagating wave along the photon ring. In this paper, we leverage the azimuthal distribution of electric vector position angle measured by the Event Horizon Telescope to study the axion-photon coupling. We propose a novel differential analysis procedure to reduce the astrophysical background, and derive stringent constraints on the existence of axions in the previously unexplored mass window of similar to(10(-21)-10(-20)) eV.

Automated summary

The unprecedented angular resolution of the Event Horizon Telescope has opened up exciting opportunities in the search for new physics. Recent measurements of linear polarization of radiation emitted near the supermassive black hole M87* have allowed for tests of the existence of a dense axion cloud produced by a spinning black hole. By analyzing the azimuthal distribution of electric vector position angle, the researchers were able to study axion-photon coupling and derive stringent constraints on the existence of axions in a previously unexplored mass range of approximately (10(-21)-10(-20)) eV.