Axion Emission Can Explain a New Hard X-Ray Excess from Nearby Isolated Neutron Stars
Malte Buschmann, Raymond T. Co, Christopher Dessert, Benjamin R. Safdi
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 $\ensuremath{\sim}100\text{ }\text{ }\mathrm{eV}$. 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 axionlike particle with mass ${m}_{a}\ensuremath{\lesssim}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\text{ }\text{ }\mathrm{eV}$ and ${g}_{a\ensuremath{\gamma}\ensuremath{\gamma}}\ifmmode\times\else\texttimes\fi{}{g}_{\text{ann}}\ensuremath{\in}\phantom{\rule{0ex}{0ex}}(2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21},{10}^{\ensuremath{-}18})\text{ }\text{ }{\mathrm{GeV}}^{\ensuremath{-}1}$ at 95% confidence, accounting for both statistical and theoretical uncertainties, where ${g}_{a\ensuremath{\gamma}\ensuremath{\gamma}}$ (${g}_{\text{ann}}$) is the axion-photon (axion-neutron) coupling constant.