The Tibet-$\mathrm{AS}\gamma$ Collaboration has recently reported a measurement of diffuse $\gamma$-ray flux from the outer Galactic disk in the energy range reaching PeV. We complement this measurement with the Fermi/LAT measurement of the diffuse flux from the same sky region and study the pion decay model of the combined $\mathrm{Fermi}/\mathrm{LA}\mathrm{T}+\mathrm{Tibet}\text{−}\mathrm{AS}\gamma$ spectrum. We find that within such a model the average cosmic-ray spectrum in the outer Galactic disk has the same characteristic features as the local cosmic-ray spectrum. In particular, it experiences a hardening at several hundred GV rigidity and a knee feature in the PV rigidity range. The slope of the average cosmic-ray spectrum above the break is close to the locally observed slope of the helium spectrum $\gamma \simeq 2.5$, but is harder than the slope of the local proton spectrum in the same rigidity range. Although the combination of Fermi/LAT and Tibet-$\mathrm{AS}\gamma$ data points to the presence of the knee in the average cosmic-ray spectrum, the quality of the data is not yet sufficient for the study of knee shape and cosmic-ray composition.

• Received 7 May 2021
• Accepted 16 July 2021

DOI:https://doi.org/10.1103/PhysRevD.104.043010