Equations describing the transport of cosmic rays in relativistic flows in the diffusion
approximation are obtained. The analysis is based on the zeroth, first, and second
differential moment equations of the relativistic Boltzmann equation with a BGK collision
term. A perturbation solution of the moment equations in the diffusion approximation yields
both the co-moving frame particle current and viscous stresses. The resultant cosmic-ray
continuity equation contains three readily recognized energy change terms: the adiabatic …