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

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