Observing axions through photon ring dimming of black holes

TL;DR

This paper proposes a method to detect axions by observing dimming in the photon ring of black holes caused by photon-axion conversion in magnetic fields, offering a new astrophysical probe for axion detection.

Contribution

It introduces a novel observational technique using photon ring dimming around black holes to detect axions, linking black hole physics with particle physics.

Findings

Photon ring dimming can reach up to 25% for supermassive black holes.

A 10% dimming in M87* could be observed with high angular resolution in X-ray and gamma-ray bands.

The dimming magnitude depends on axion-photon coupling and axion mass.

Abstract

It is known that magnetic fields exist near black holes and photons can go around black holes due to strong gravity. Utilizing these facts, we can probe hypothetical pseudoscalar particles, so-called axions. In fact, photons can be converted into axions when they propagate in a magnetic field. The conversion of such photons into axions leads to a dimming of the photon ring around the black hole shadow. We show that photon ring dimming can occur efficiently for supermassive black holes. Remarkably, it turns out that the maximal dimming rate of the photon ring is 25%. In the case of M87*, the dimming of 10% will be observed in the X-ray and gamma-ray bands if the angular resolution of $10^{-5}$ arcsec is achieved. The frequency band and the magnitude of the dimming depend on the axion-photon coupling and axion mass. Hence, the distorted spectrum of the photon ring provides a novel tool for detecting axions.