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

Abstract

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 σ . 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 $m_a=(3.6\begin{array}{c}+0.5_{stat.}\\ -0.2_{stat.}\end{array}\pm 0.2_{syst.})$ neV and $g_{a\gamma \gamma }=(2.3\begin{array}{c}+0.3_{stat.}\\ -0.4_{stat.}\end{array}\pm 0.4_{syst.})\times 10^{-10}$ 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.

19 pages, 22 figures, Prepared for submission to JCAP

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