Testing the ALP-photon coupling with polarization measurements of Sagittarius A*

TL;DR

This study uses polarization measurements of radiation from Sagittarius A* to set new upper limits on the coupling between axion-like particles and photons, improving constraints on ultra-light ALPs in a specific mass range.

Contribution

The paper introduces a novel method of using high-resolution polarimetric data from the Event Horizon Telescope to constrain ALP-photon coupling, providing stronger limits than previous astrophysical observations.

Findings

Derived upper limits on ALP-photon coupling of ~10^{-12} GeV^{-1}

Constraints are stronger than those from SN1987A and supernovae in the same mass range

Future measurements could further improve these constraints.

Abstract

Ultra-light bosons such as axions or axion-like particles (ALPs), are promising candidates to solve the dark matter problem. A unique way to detect such ALPs is to search for the periodic oscillation feature of the position angles of linearly polarized photons emitted from the regions where ALP dark matter forms a solitonic core. In this work, we use the high-resolution polarimetric measurements of the radiation near the super-massive black hole (SMBH) in the center of the Milky Way, i.e., Sagittarius A$^\star$ (Sgr A$^\star$), by a sub-array of the Event Horizon Telescope to search for the ultra-light ALPs. We derive upper limits on the ALP-photon coupling of $\sim 10^{-12}{\rm GeV^{-1}}$ for ALP masses of $m\sim (10^{-19}-10^{-18})$eV. Our results are stronger than that derived from the observations of SN1987A and a population of supernovae in the mass window of ($10^{-19}-10^{-17}$)eV. Improved polarimetric measurements with the full Event Horizon Telescope can further strengthen the constraints.