A higher-order Godunov method for the radiation subsystem of radiation
hydrodynamics is presented. A key ingredient of the method is the direct coupling of
stiff source term effects to the hyperbolic structure of the system of conservation
laws; it is composed of a predictor step that is based on Duhamel’s principle and a
corrector step that is based on Picard iteration. The method is second-order accurate
in both time and space, unsplit, asymptotically preserving, and uniformly well
behaved from the photon free streaming (hyperbolic) limit through the weak
equilibrium diffusion (parabolic) limit and to the strong equilibrium diffusion
(hyperbolic) limit. Numerical tests demonstrate second-order convergence across
various parameter regimes.