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Tibet-AS$\gamma$ collaboration has recently reported a measurement of diffuse$\gamma$-ray flux from the outer Galactic disk in the energy range reachingPeV. We complement this measurement with the Fermi/LAT measurement of thediffuse flux from the same sky region and study the pion decay model of thecombined Fermi/LAT+Tibet-AS$\gamma$ spectrum. We find that within such a modelthe average cosmic ray spectrum in the outer Galactic disk has the samecharacteristic features as the local cosmic ray spectrum. In particular, itexperiences a hardening at several hundred GV rigidity and a knee feature inthe PV rigidity range. The slope of the average cosmic ray spectrum above thebreak 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 spectrumin the same rigidity range. Although the combination of Fermi/LAT andTibet-AS$\gamma$ data points to the presence of the knee in the average cosmicray spectrum, the quality of the data is not yet sufficient for the study ofknee shape and cosmic ray composition.

Pion decay model of TIBET-AS$\gamma$ PeV gamma-ray signal