Cosmic-Ray Source Grammage Dominates The Diffuse Gamma-Ray Sky

Recent secondary-over-primary cosmic-ray (CR) ratio measurements by DAMPE and CALET show a hint of a flattening above $( \sim)$ TV rigidities. It is plausible - and theoretically well-motivated - that CRs accumulate additional grammage inside the source environment leading to a constant grammage in addition to the Galactic one. In this contribution, we explore this scenario, quantifying the contribution of these cocoon regions onto the secondary diffuse emissions such as gamma-rays and neutrinos. Interestingly, we find that, assuming a source grammage of $\sim 0.4 \frac{\rm gr}{\rm cm^2}$, compatible with the grammage accumulated by CRs in the downstream regions of Supernova Remnant shocks, and fitted against high-energy B/C measurements, the corresponding gamma-ray emission must substantially contribute to the diffuse gamma-ray flux providing a natural explanation of the gamma-ray hardening observed in the inner Galaxy. Our results may also additionally reproduce the recently published TeV-PeV gamma-ray measurements from the LHAASO and Tibet observations. We also discuss the implications of our calculation for the neutrino fluxes in light of the recent Galactic neutrino flux measured by the IceCube collaboration.