Black hole photon rings could reveal invisible axion particles

Photons orbiting close to a black hole travel along extended near-circular paths before escaping, and this extra path length dramatically boosts the chance that they convert into axions in the presence of a magnetic field. Around M87*, the effect is strongest at X-ray and gamma-ray frequencies, and a spinning black hole dims more than a static one. Future X-ray telescopes with arcsecond-scale resolution could use this dimming to pin down axion mass and its coupling to light.