  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.
