Axion Emission Can Explain a New Hard $X$-ray Excess from Nearby Isolated Neutron Stars

TL;DR

This paper proposes that axion-like particles could explain the unexplained hard X-ray excess observed in nearby isolated neutron stars, providing a novel astrophysical signature for axions.

Contribution

It introduces a new mechanism where axions produced in neutron star cores convert into X-rays, explaining the observed excess without existing astrophysical explanations.

Findings

Hard X-ray excess can be explained by axion-like particles.

Constraints on axion mass and couplings are derived from observations.

No conventional astrophysical explanation currently accounts for the excess.

Abstract

Axions may be produced thermally inside the cores of neutron stars (NSs), escape the stars due to their feeble interactions with matter, and subsequently convert into X-rays in the magnetic fields surrounding the stars. We show that a recently-discovered excess of hard X-ray emission in the 2 - 8 keV energy range from the nearby Magnificent Seven isolated NSs could be explained by this emission mechanism. These NSs are unique in that they had previously been expected to only produce observable flux in the UV and soft X-ray bands from thermal surface emission at temperatures ~100 eV. No conventional astrophysical explanation of the Magnificent Seven hard X-ray excess exists at present. We show that the hard X-ray excess may be consistently explained by an axion-like particle with mass $m_a \lesssim 2 \times 10^{-5}$ eV and $g_{a\gamma\gamma} \times g_{ann} \in (2 \times 10^{-21}, 10^{-18})$ GeV$^{-1}$ at 95\% confidence, accounting for both statistical and theoretical uncertainties, where $g_{a\gamma\gamma}$ ($g_{ann}$) is the axion-photon (axion-neutron) coupling constant.