Search for Axion-Like Particles in High-Magnetic-Field Pulsars with NICER

Axion-like particles (ALPs) can couple to photons in strong magnetic fields, producing characteristic fluctuations in X-ray spectra. Using data from NASA’s Neutron Star Interior Composition Explorer (NICER), we analyzed three pulsars: PSR J2229+6114, PSR J1849-0001, and PSR B0531+21, to search for such features. Each spectrum was modeled with a sliding-window power-law fitting method to identify local deviations from the smooth continuum. From these analyses, we derived constraints on the axion-photon coupling constant $g_{a\gamma \gamma }$ assuming dipole and quadrupole magnetic field distributions. We obtain upper limits in the range ${10}^{-10}$–${10}^{-13}{\mathrm{G}\mathrm{e}\mathrm{V}}^{-1}$ for ALP masses in the $0.8$–$10\mathrm{k}\mathrm{e}\mathrm{V}$ region.