Gamma-ray spectral modulations of Galactic pulsars caused by photon-ALPs mixing

Well-motivated extensions of the standard model predict ultra-light and fundamental pseudo-scalar particles (e.g., axions or axion-like particles: ALPs). Similarly to the Primakoff-effect for axions, ALPs can mix with photons and consequently be searched for in laboratory experiments and with astrophysical observations. Here, we search for energy-dependent modulations of high-energy gamma-ray spectra that are tell-tale signatures of photon-ALPs mixing. To this end, we analyze the data recorded with the Fermi -LAT from Galactic pulsars selected to have a line of sight crossing spiral arms at a large pitch angle. The large-scale Galactic magnetic field traces the shape of spiral arms, such that a sizable photon-ALP conversion probability is expected for the sources considered. In nine years of Fermi -LAT data, we detect significant spectral features in the selected source-sample consistent with photon-ALPs oscillation with a combined statistical significance of 5.52 {\sigma} . Notably, sources with neighboring lines of sight share similar spectral features. From a common fit to all sources, we determine the most-likely parameters for mass $m_{a}$ and coupling $g_{a\gamma\gamma}$ to be $m_a=(3.6 \substack{+0.5_ \mathrm{stat.}\\-0.2_ \mathrm{stat.}}\pm 0.2_\mathrm{syst.} )$ neV and $g_{a\gamma\gamma}=(2.3\substack{+0.3_ \mathrm{stat.}\\-0.4_ \mathrm{stat.}}\pm 0.4_\mathrm{syst.})\times 10^{-10}$ GeV$^{-1}$. In the error budget, we consider instrumental effects, scaling of the adopted Galactic magnetic field model ($\pm~20~\%$), and uncertainties on the distance of individual sources. We note that an astrophysical interpretation of the detected modulation is not obvious.