Stringent axion constraints with Event Horizon Telescope polarimetric measurements of M87$^\star$

TL;DR

This paper uses Event Horizon Telescope polarimetric data of M87$^ ext{star}$ to set new constraints on axion-like particles by analyzing polarization oscillations near the black hole.

Contribution

It introduces a novel differential analysis method to constrain axion-photon coupling using EHT polarization measurements, exploring a new mass window.

Findings

Set stringent limits on axion-photon coupling for masses ~10^{-21} to 10^{-20} eV.

Demonstrated the effectiveness of polarization-based tests for new physics.

Reduced astrophysical background in axion searches with a novel analysis technique.

Abstract

The hitherto unprecedented angular resolution of the Event Horizon Telescope (EHT) has created exciting opportunities in the search for new physics. Recently, the linear polarization of radiation emitted near the supermassive black hole M87$^\star$ was measured on four separate days, precisely enabling tests of the existence of a dense axion cloud produced by a spinning black hole. The presence of an axion cloud leads to a frequency-independent oscillation in the electric vector position angle (EVPA) of this linear polarization. For a nearly face-on M87$^\star$, this oscillation in the EVPA appears as a propagating wave along the photon ring. In this paper, we leverage the azimuthal distribution of EVPA measured by the EHT to study the axion-photon coupling. We propose a novel differential analysis procedure to reduce the astrophysical background, and derive stringent constraints on the existence of axions in the previously unexplored mass window $\sim (10^{-21}-10^{-20})$~eV.