X-Ray Searches for Axions from Super Star Clusters

Axions may be produced in abundance inside stellar cores and then convert into observable x rays in the Galactic magnetic fields. We focus on the Quintuplet and Westerlund 1 super star clusters, which host large numbers of hot, young stars including Wolf-Rayet stars; these stars produce axions efficiently through the axion-photon coupling. We use Galactic magnetic field models to calculate the expected x-ray flux locally from axions emitted from these clusters. We then combine the axion model predictions with archival Nuclear Spectroscopic Telescope Array (NuSTAR) data from 10–80 keV to search for evidence of axions. We find no significant evidence for axions and constrain the axion-photon coupling $g_{a\gamma \gamma }\lesssim 3.6\times {10}^{-12}{\mathrm{GeV}}^{-1}$ for masses $m_a\lesssim 5\times {10}^{-11}\mathrm{eV}$ at 95% confidence.

Figure 1

The stacked and pixelated background-subtracted count data (10–80 keV) from the NuSTAR observations of the Quintuplet SSC. The locations of the stars are indicated in black, while the 90% energy containment region for emission associated with the SSC is indicated by the black circle, accounting for the NuSTAR point spread function (PSF). ${\mathrm{RA}}_0$ and ${\mathrm{DEC}}_0$ denote the locations of the cluster center. We find no evidence for axion-induced emission from this SSC, which would follow the spatial counts template illustrated in the inset panel.

Figure 2

The spectra associated with the axion-induced templates from the Quintuplet and Wd1 SSCs constructed from the NuSTAR data analyses, with best-fit points and $1\sigma$ uncertainties indicated. In red we show the predicted spectra from an axion with $m_a\ll {10}^{-11}\mathrm{eV}$ and indicated $g_{a\gamma \gamma }$. Note that for Wd1 we do not analyze the 10–15 keV energy bin because of ghost-ray contamination.

Figure 3

The 95% upper limits (black) on $g_{a\gamma \gamma }$ as a function of the axion mass from the Quintuplet and Wd1 data analyses. We compare the limits to the $1\sigma$ (green band) and $2\sigma$ (yellow band) expectations under the null hypothesis, along with the median expectations (dotted). The joint 95% upper limit, combining Quintuplet and Wd1, is also indicated (expected joint limit not shown). At low masses our limits may be surpassed by those from searches for x-ray spectral modulations from NGC 1275 [76], though we caution that those limits have been called into question recently, as discussed further in the text [77].