2025ApJ...981...93M

We use the GALPROP cosmic ray (CR) framework to model the Galactic CR distributions and associated nonthermal diffuse emissions up to PeV energies. We consider ensembles of discrete, finite lifetime CR sources, e.g., supernova remnants, for a range of creation rates and lifetimes. We find that the global properties of the CR sources are likely not directly recoverable from the current “snapshot” of the historic injection and propagation of CRs within the Galaxy that are provided by the data. We show that models for the diffuse γ rays based on the discrete/time-dependent scenarios we consider are able to explain the LHAASO very-/ultra-high-energy (VHE/UHE) γ-ray data with up to 50% contribution by unresolved leptonic sources at the highest energies. Over the models that we consider, variations in the diffuse VHE emissions can be ∼25%, which is comparable to those for the steady-state models that we investigated in an earlier work. Such variations due to the discrete/finite nature of the CR sources are an important factor that are necessary to construct accurate physical models of the diffuse emissions from the Galaxy at VHE/UHEs.