The double Compton process in astrophysical plasmas

We study the double Compton (DC) process for a wide range of particle energies, extending previous treatments well beyond the soft photon limit, employing both numerical and analytical methods. This allows us to investigate the physics of the DC process up to the highly relativistic regime relevant to electromagnetic particle cascades in the early Universe and photon-dominated astrophysical plasmas. Generalized exact analytic expressions for the DC emissivity in the soft photon limit are obtained. These are compared to existing approximations, for the first time studying the ultra-relativistic regime. We also numerically integrate the full DC collision term calculating the DC emissivity at general particle energies. A careful treatment of DC infrared divergences inside astrophysical plasmas, including subtle effects related to the presence of stimulated DC emission, is discussed. The obtained results can be efficiently represented using the code DCpack, which also allows one to compute average emissivities for general incoming electron and photon distributions. This puts the modelling of the DC process inside astrophysical plasmas on a solid footing and should find applications in particular for computations of the cosmological thermalization problem in the early Universe.