FAST Observations of Two Neutron Stars Yield Tightest ACL-Based Limits on Axion-Photon Coupling Below 6.2 μeV

The search exploits the Primakoff conversion of axion dark matter into narrow spectral lines inside the magnetospheres of strongly magnetized, radio-quiet neutron stars. Two X-ray dim isolated neutron stars were chosen because their spin-down power and magnetic geometry maximize the expected signal within FAST’s declination range. With 5.3 hours on-source integration and careful removal of terrestrial RFI, the team reached a 5σ sensitivity sufficient to surpass all prior neutron-star conversion limits in this mass window. The non-detection directly constrains the QCD axion band that remains inaccessible to current haloscope experiments such as ADMX, whose cavity geometry limits coverage above ~ few μeV. Because the conversion occurs in a natural plasma-frequency gradient rather than an artificial resonator, the astrophysical bound is independent of the laboratory cavity assumptions that dominate existing exclusions. Systematic uncertainties in the neutron-star magnetic-field geometry and local dark-matter density remain the dominant limitation; deeper integration or additional targets with better-constrained magnetospheres would tighten the limit by another factor of two to three. Future FAST campaigns targeting a larger sample of XDINS objects, combined with SKA-mid’s higher sensitivity, could either discover the line or push the coupling below the canonical KSVZ/DFSZ predictions, providing a decisive test of the axion as dark matter.