Stochastic modelling of cosmic-ray sources for Galactic diffuse emissions

Abstract

Galactic diffuse emissions in gamma rays and neutrinos arise from interactions of cosmic rays with the interstellar medium and probe the cosmic-ray intensity away from the Solar system. Model predictions for those are influenced by the properties of cosmic-ray sources, and understanding the impact of cosmic-ray sources on Galactic diffuse emissions is key for interpreting measurements by LHAASO, Tibet AS-gamma, IceCube, and the upcoming SWGO. We consider supernova remnants as prototypical cosmic-ray sources and study the impact of their discreteness on the Galactic diffuse emissions in different source injection and near-source transport models in a stochastic Monte Carlo study. Three lessons exemplify the results of our simulations: First, the distributions of Galactic diffuse emission intensities can be described by a mixture model of stable laws and Gaussian distributions. Second, the maximal deviations caused by discrete sources across the sky depend on energy, reaching typically tens of percent in burst-like and energy-dependent escape scenarios but order unity or larger in a time-dependent diffusion scenario. Third, the additional model uncertainty from source stochasticity is subdominant in burst-like and energy-dependent escape scenarios, but becomes sizeable above some tens of TeV in the time-dependent diffusion scenario, where it can help reconcile model predictions with LHAASO measurements. With increased spatial resolution, especially at energies beyond tens of TeV, measurements of Galactic diffuse emissions can be expected to constrain source models and locate cosmic ray sources.