Researchers have explored the conversion of photons into axions in the vicinity of rotating Kerr black holes, a phenomenon that could manifest as a dimming of the spectral luminosity of the photon ring. This process is favored by the intense gravity of these objects, which traps photons in nearly circular trajectories, significantly increasing their effective path length. Photon-axion conversion, driven by ambient magnetic fields, is predicted to be particularly efficient around supermassive black holes like M87*, where photon luminosity scales with the black hole's mass.

The study analyzes how various parameters influence the conversion probability and the consequent dimming of spectral luminosity. These include photon frequency, axion mass, photon-axion coupling, magnetic field strength, plasma density, and black hole spin. The results indicate that conversion is more efficient at high frequencies, such as X-rays and gamma rays. Furthermore, the frequency window for efficient conversion broadens with stronger photon-axion coupling and narrows with lower electron density and lower axion mass. The magnitude of the spectral luminosity dimming primarily depends on the magnetic field, photon-axion coupling, and black hole spin; rotating black holes show amplified dimming compared to static ones.

This work suggests that future observations with high-resolution telescopes, approximately 10⁻⁵ arcseconds in the X-ray/gamma-ray band, could detect this dimming. If confirmed, such measurements would provide valuable constraints on the axion mass and its coupling to photons. Axions are hypothetical particles that could solve the strong CP problem in quantum chromodynamics and are candidates for dark matter. Detecting this effect in photon rings would offer a unique experimental avenue to search for these elusive particles in extreme astrophysical environments.