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 $\sim 100\mathrm{eV}$. 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 $m_a\lesssim 2\times {10}^{-5}\mathrm{eV}$ and $g_{a\gamma \gamma }\times g_{\text{ann}}\in (2\times {10}^{-21},{10}^{-18}){\mathrm{GeV}}^{-1}$ at 95% confidence, accounting for both statistical and theoretical uncertainties, where $g_{a\gamma \gamma }$ ($g_{\text{ann}}$) is the axion-photon (axion-neutron) coupling constant.

中文翻译：

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轴距发射可以解释附近孤立的中子星产生的新的硬X射线过量

在中子星（NSs）的核内部可能会产生热轴，由于它们与物质的微弱相互作用而使恒星逃逸，然后在围绕恒星的磁场中转换为x射线。我们表明，最近发现的2-8 keV能量范围内的硬X射线过量发射是由附近的7个孤立NS组成，这可以用这种发射机制来解释。这些NS的独特之处在于，以前曾预计它们仅会因温度下的热表面发射而在UV和软X射线带中产生可观察到的通量$〜100\mathrm{电子伏特}$。目前还没有常规的天体物理学解释来解释巨大的7次硬X射线过剩。我们表明，硬质X射线过剩可以用质量类似的轴状颗粒一致地解释${米}_{\mathrm{一种}}\lesssim 2\times {10}^{-5}\mathrm{电子伏特}$ 和 $G_{\mathrm{一种}\gamma \gamma }\times G_{\text{安}}\in （2\times {10}^{-21}，{10}^{-\mathrm{18岁}}）{\mathrm{GeV}}^{-\mathrm{1个}}$ 在95％的置信度下，考虑了统计和理论上的不确定性， $G_{\mathrm{一种}\gamma \gamma }$ （$G_{\text{安}}$）是轴-光子（轴-中子）的耦合常数。