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

Abstract

Compton scattering exchanges energy between radiation and hot gas and changes the spectrum in high-energy astrophysical systems such as tidal disruption events (TDEs). Because it couples all photon energies and can act faster than the hydrodynamic time step, many radiation-hydrodynamics (RHD) simulations omit its spectral redistribution or treat its energy exchange in the gray approximation. We extend the multigroup flux-limited diffusion module of the moving-mesh RHD code RICH to include Compton scattering with the exact thermal Compton kernel. In our semi-implicit scheme for the thermal radiative transfer equation, the temperature dependence of the kernel enters a generalized Fleck factor, and the group Compton operator conserves photon number and satisfies detailed balance. In equilibration tests, the scheme agrees with independent solutions, relaxes without absorption and emission to a Bose-Einstein spectrum, and remains stable for time steps that do not resolve the Compton equilibration timescale. We apply it to a three-dimensional simulation of the disruption of a $0.5\,M_\odot$ star by a $10^6\,M_\odot$ black hole. We find that during the X-ray flash, inverse Compton scattering dominates the coupling between the gas and the radiation near the nozzle shock, where it adds X-ray energy much faster than thermal emission and may affect the dynamics. Outside the X-ray thermalization radii, its instantaneous X-ray production rate during the flash is comparable to or larger than the escaping X-ray luminosity, so it may contribute substantially to the X-ray emission from TDEs.