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 γ γ}≲3.6 ×10^-12 GeV^-1 for masses m_a≲5 ×10^-11 eV at 95% confidence.