Implications of the detection of sub-PeV diffuse $\gamma$ rays from the Galactic disk apart from discrete sources

Abstract

Very recently, the Tibet-AS$\gamma$ collaboration reported the detection of $\gamma$ rays from the galactic disk in the energy range of 100 TeV -- 1 PeV. Remarkably, many of these $\gamma$ rays were observed apart from known very high energy (E$>$ 100 GeV) $\gamma$-ray sources. These results are best understood if these diffuse $\gamma$ rays: 1) were produced by a conventional rather than an exotic (i.e. dark matter decay or annihilation) process, 2) have a hadronic rather than a leptonic origin, 3) were produced in impulsive rather than stable sources or, alternatively, in optically thick sources. In addition to that, the detection of the sub-PeV diffuse $\gamma$ rays implies a limit on the flux of neutrinos from the Galactic disk and a lower limit on the rigidity of the cutoff in the Galactic cosmic ray spectrum.