Modelling Galactic CRs and the Diffuse Gamma-Ray Emission

Galactic diffuse gamma-ray emissions have been observed from MeV to PeV energies. These emissions are connected via a common origin of the cosmic ray (CR) particles, but the energy dependence and hadronic/leptonic fraction remain unconstrained. We model the Galactic CR distributions and associated non-thermal diffuse emissions from TeV-PeV energies using the GALPROP framework. We investigate the modelling uncertainty over a grid of steady-state 3D models that include variations over the ISM target distributions and the CR source locations. We also estimate the time-dependent modelling uncertainties when considering ensembles of CR sources with discrete and finite lifetime, e.g. supernova remnants, for a range of creation rates and active times. Our predictions are compared to diffuse emission estimates from the high-energy stereoscopic system (H.E.S.S.) and large high altitude air shower observatory (LHAASO) observations after accounting for resolved and unresolved source fluxes. We show that these $>$TeV observations can be described by our models tuned at GeV energies without any alterations to the underlying physics and assumptions. Over the range of distributions we consider, variations in the diffuse VHE emissions can be $\sim$25% for both the steady-state and time-dependent models. The variations due to the differing descriptions of the ISM are an important factor that must be reduced through other multi-messenger constraints on the MW. The variations due to the discrete/finite nature of the CR sources are critical to consider when describing physical models of the diffuse emissions from the Galaxy at VHE/UHEs.