  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.
