Observing axions through photon ring dimming of black holes

It is known that magnetic fields exist near black holes and photons can go around due to strong gravity. Utilizing these facts, we probe hypothetical pseudoscalar particles, so-called axions. In fact, be converted into axions when they propagate in a field. The conversion of such leads dimming the photon ring hole shadow. We show occur efficiently for supermassive holes. Remarkably, it turns out maximal rate 25%. case M87*, 10% will observed X-ray gamma-ray bands if angular resolution $10^{-5}$ arcsec achieved. frequency band magnitude depend on axion-photon coupling axion mass. Hence, distorted spectrum provides novel tool detecting