Photon ring dimming as a signature of photon-axion conversion in Janis-Newman-Winicour naked singularity

The possible existence of axions in the Universe introduces the intriguing possibility of photon-axion conversion in strong magnetic fields, particularly near compact objects like supermassive black holes or even naked singularity. In this study, we investigate the conversion of photons into axions in the vicinity of a Janis-Newman-Winicour (JNW) spacetime, a well-known naked singularity solution. We calculate the conversion probability and find that it is significantly influenced by the characteristic parameter of the JNW spacetime. The potential observational signatures of this conversion would be the dimming of photon ring in the x-ray and gamma-ray spectrum. Our findings suggest that compact objects like M87* could be prime candidates for detecting photon-axion conversion effects, provided future advances in high-resolution observations. Our analysis also suggests that the scattering of photons during the propagation through plasma is insignificant for the estimation of the conversion probability.