# Probing Axions with Event Horizon Telescope Polarimetric Measurements

## TL;DR

High-resolution polarimetric imaging of supermassive black holes can detect axion-like particles by observing polarization oscillations caused by axion-induced birefringence, providing a novel probe of ultralight bosons.

## Contribution

This work proposes using Event Horizon Telescope polarimetric data to detect axions via birefringence effects, offering a new method to explore axion properties around black holes.

## Key findings

- Can probe axion-photon coupling for masses around 10^{-20} eV and 10^{-17} eV.
- Sensitive to axion decay constants below 10^{16} GeV.
- Complementary to existing axion detection methods.

## Abstract

With high spatial resolution, polarimetric imaging of a supermassive black hole, like M87$^\star$ or Sgr A$^\star$, by the Event Horizon Telescope can be used to probe the existence of ultralight bosonic particles, such as axions. Such particles can accumulate around a rotating black hole through the superradiance mechanism, forming an axion cloud. When linearly polarized photons are emitted from an accretion disk near the horizon, their position angles oscillate due to the birefringent effect when traveling through the axion background. In particular, the observations of supermassive black holes M87$^\star$ (Sgr A$^\star$) can probe the dimensionless axion-photon coupling $c = 2 \pi g_{a \gamma} f_a$ for axions with mass around $O(10^{-20})$~eV ($O( 10^{-17}$)~eV) and decay constant $f_a < O(10^{16})$ GeV, which is complimentary to other axion measurements.