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 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.

Figure 1

The energy spectrum from 2 to 8 keV for NS J1856 as measured by combining PN, MOS, and Chandra data, with 68% statistical uncertainties [11]. We also show the best-fit axion model spectrum from a fit to this NS only, with the core temperature fixed to the central value in Table 1.

Figure 2

95% exclusion limit and best fit 1 and $2\sigma$ regions from a joint likelihood analysis over all of the M7 and combining PN, MOS, and Chandra data. We compare our result to existing limits from $\mathrm{CAST}2017+\mathrm{NS}$ cooling. All curves and regions continue to arbitrarily small $m_a$. Note that the QCD axion model is too weakly coupled to appear in this figure. Accounting for systematic uncertainties may allow for smaller values of $g_{a\gamma \gamma }{g}_{\text{ann}}$, by approximately an order of magnitude, as discussed in the Supplemental Material [64].

Figure 3

Best-fit intensities $I_{2–8}$ and $I_{4–8}$ for all M7. The green (yellow) bands indicate the 68% (95%) confidence intervals from the x-ray intensity measurements, with best-fit intensities marked by vertical green lines. Black and gray error bands denote the 68% and 95% confidence intervals for the axion model predictions at the global best-fit coupling $g_{a\gamma \gamma }{g}_{\text{ann}}$ and $m_a\ll {10}^{-5}\mathrm{eV}$, with uncertainties arising from uncertain aspects of the NSs.

Figure 4

As in Fig. 3 but for the best-fit core temperature $T_b^{\infty }$ for J1856 and J0420.