First Constraint on Axion-Photon Coupling $g_{\gamma }$ from Neutron Star Observations

We propose a novel method to detect axions which uniquely depends on the dimensionless axion-photon coupling $g_{\gamma }$, independent of the suppressive axion decay constant $f_a$. Using neutron star PSR B1919+21 data from the Five-hundred-meter Aperture Spherical Telescope, we derive the first constraint $\mathrm{\mid }{g}_{\gamma }\mathrm{\mid }<0.93$ at $1\sigma$ confidence level for ultra-light axions ($m_a<{10}^{-11}$ eV).