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

TL;DR

This study examines how photon-axion conversion near Kerr black holes can cause observable dimming of high-frequency photon emissions, offering potential constraints on axion properties.

Contribution

It provides a detailed numerical analysis of photon-axion conversion effects around rotating black holes, highlighting conditions for efficient dimming in high-energy bands.

Findings

Conversion is most efficient at high photon frequencies (X-rays, gamma rays).

Dimming magnitude depends on magnetic field, photon-axion coupling, and black hole spin.

Rotating black holes show enhanced dimming compared to static ones.

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.