Multigroup Radiation Diffusion on a Moving Mesh: Implementation in RICH and Application to Tidal Disruption Events

Radiation hydrodynamics (RHD) determines the bulk evolution and observable emission in a wide variety of high-energy astrophysical phenomena. Due to their complexity, RHD problems must usually be studied through numerical simulations. We have extended the publicly available RICH code, which previously solved the RHD equations in the limit of gray flux-limited diffusion (FLD), to operate with a multigroup FLD solver. RICH is a semi-Lagrangian code that solves the RHD equations on an unstructured moving mesh, and represents the first multigroup RHD moving-mesh code, making it uniquely suited to problems with extreme dynamic range and dynamically important radiation forces. We validate our multigroup module against multiple analytic benchmarks, including a novel test of the RHD Doppler term. The computational efficiency of the code is enhanced by a novel scheme that accelerates convergence in optically thick cells by limiting the absorption coefficients. Finally, we apply multigroup RICH in a pilot three-dimensional study of a stellar tidal disruption event (TDE) involving a 10⁴ M_☉ intermediate-mass black hole. Our simulations self-consistently produce a bright early-time X-ray flash prior to peak optical/UV light, in qualitative agreement with postprocessed (gray) RICH simulations of supermassive black hole TDEs, as well as X-ray observations of the TDE AT 2022dsb.