Using the 3D simulation software GALPROP, we modelled the Galactic cosmic ray (CR) diffusion and investigated the time variability of the gamma-ray flux along the Galactic plane using a distribution of stochastically placed CR sources. These CR sources more accurately represent the formation rate and finite lifetimes compared to the steady-state CR injections models that are typically assumed.

Our results show that the leptonic component of the gamma-ray emission is highly sensitive to the assumed electron injection and spectral characteristics. Furthermore, the leptonic component is heavily dependent on the positions of the sources due to the rapid synchrotron cooling of the very-high-energy electrons. At 1 TeV the total gamma-ray flux along the Galactic plane can vary by as much 50% due only to the stochasticity of the CR source placement. Therefore, the large-scale gamma-ray emission that CTA will observe may be significantly influenced by gamma rays local to CR accelerators.

The diffuse emission that CTA observes will be heavily impacted by the gamma-ray variability, with the variability in the large-scale gamma-ray emission having important implications for any background modelling that CTA performs. Hence, we will also provide the first look at the time-dependent morphology in the multi-TeV gamma-ray structures in the Milky Way and quantify the variation over time and Galactic longitude.