# A New Bound on Axion-Like Particles

## TL;DR

This paper sets new upper limits on the coupling strength of axion-like particles to photons by analyzing X-ray spectra from M87, constraining ALP properties and informing future experimental searches.

## Contribution

It provides the first astrophysical constraints on ALP-photon coupling using deep Chandra X-ray observations of M87, improving existing bounds and guiding future experiments.

## Key findings

- Established an upper limit of $g_{a\gamma} < 2.6\times10^{-12}$ GeV$^{-1}$ for ALP masses $m_a \leq 10^{-13}$ eV.
- Stronger limits of $g_{a\gamma} \lesssim 1.1$--$1.5 \times 10^{-12}$ GeV$^{-1}$ were obtained with alternative magnetic field models.
- The bounds surpass previous constraints from SN1987A and impact future ALP searches.

## Abstract

Axion-like particles (ALPs) and photons can quantum mechanically interconvert when propagating through magnetic fields, and ALP-photon conversion may induce oscillatory features in the spectra of astrophysical sources. We use deep (370 ks), short frame time Chandra observations of the bright nucleus at the centre of the radio galaxy M87 in the Virgo cluster to search for signatures of light ALPs. The absence of substantial irregularities in the X-ray power-law spectrum leads to a new upper limit on the photon-ALP coupling, $g_{a\gamma}$: using a conservative model of the cluster magnetic field consistent with Faraday rotation measurements from M87 and M84, we find $g_{a \gamma} < 2.6\times10^{-12}$ GeV$^{-1}$ at 95% confidence level for ALP masses $m_a \leq 10^{-13}$ eV. Other consistent magnetic field models lead to stronger limits of $g_{a \gamma} \lesssim 1.1$--$1.5 \times 10^{-12}$ GeV$^{-1}$. These bounds are all stronger than the limit inferred from the absence of a gamma-ray burst from SN1987A, and rule out a substantial fraction of the parameter space accessible to future experiments such as ALPS-II and IAXO.