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

TL;DR

This paper extends the RICH radiation-hydrodynamics code to include multigroup flux-limited diffusion on a moving mesh, enabling detailed simulations of high-energy astrophysical phenomena like tidal disruption events with improved accuracy and efficiency.

Contribution

Introduction of a multigroup FLD solver in the RICH code on a moving mesh, validated against benchmarks, and applied to simulate a stellar tidal disruption event.

Findings

Successfully validated the multigroup module with analytic benchmarks.

Achieved efficient convergence in optically thick regions.

Simulated a TDE showing early X-ray flash consistent with observations.

Abstract

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 simulation. We have extended the publicly available RICH code, which previously solved the equations of RHD in the limit of grey flux-limited diffusion (FLD), to operate with a multigroup FLD solver. RICH is a semi-Lagrangian code that solves the equations of RHD on an unstructured moving mesh, and is the first multigroup RHD moving mesh code, making it uniquely applicable 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 aided by a novel scheme to accelerate convergence in optically thick cells by limiting the absorption coefficients. Finally, we apply multigroup \textsc{rich} in a pilot three dimensional study of a stellar tidal disruption event (TDE), using a $10^4 M_\odot$ 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 post-processing of (grey) RICH simulations of supermassive black hole TDEs, as well as X-ray observations of the TDE AT 2022dsb.