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

Abstract

Tibet-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 Fermi/LAT+Tibet-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-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.

5 pages, 2 figures

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• Galactic contribution to the high-energy neutrino flux found in track-like IceCube events
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• The origin of high-energy astrophysical neutrinos: new results and prospects
• Two-sided constraints on Lorentz Invariance violation from Tibet-AS$\gamma$ and LHAASO Very-High-Energy photon observations
• Hot spots in the neutrino flux created by cosmic rays from Cygnus and Vela?
• First constraints on axion-like particles from Galactic sub-PeV gamma rays
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• New limit on high Galactic latitude PeV gamma-ray flux from Tibet AS-gamma data