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arXiv · 2605.22696

Dimming of Photon Ring due to Photon-Axion Conversion around Kerr Black Holes

Rahul Dhyani·Sauvik Sen·Indrani Banerjee·Ashmita Chakraborty·Arindam Chatterjee
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Abstract

We investigate photon-axion conversion in the vicinity of rotating Kerr black holes where strong gravity traps photons on near-circular trajectories, effectively enhancing the path length. We explore the observable signatures of such a conversion near the photon region. The process, driven by ambient magnetic fields, is significantly more efficient around supermassive black holes such as M87*, since the luminosity of photons increases with the mass of the BH. By numerically evaluating photon path lengths (on which the conversion depends), we analyze how key parameters-photon frequency, axion mass, photon-axion coupling, magnetic field strength, plasma density, and black hole spin-affect the conversion probability and the resultant dimming of photon spectral luminosity. We find that the conversion is most efficient at high frequencies (X-rays and gamma rays), while the frequency window associated with efficient conversion widens with an increase in the photon-axion coupling and a decrease in the electron density and the axion mass. The magnitude of dimming of the photon spectral luminosity depends primarily on the magnetic field, the photon-axion coupling and the BH spin. Our study reveals that rotating black holes generally exhibit enhanced dimming compared to static ones. Thus, if future telescopes achieving a resolution $\sim 10^{-5}$ arcsec in the X-ray/gamma-ray band detect a dimming of the photon spectral luminosity, then they can provide interesting constraints on the axion mass and its coupling with photons.

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Rahul Dhyani, Sauvik Sen, Indrani Banerjee, Ashmita Chakraborty, Arindam Chatterjee. 2026-05-21. Dimming of Photon Ring due to Photon-Axion Conversion around Kerr Black Holes. https://arxiv.org/abs/2605.22696

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