Multi-Frequency General Relativistic Radiation-Hydrodynamics with $\bf{M}_1$ Closure

TL;DR

This paper presents enhancements to the Cosmos++ code, enabling multi-frequency, general relativistic radiation transport with Doppler and gravitational effects, validated through diverse tests across different spacetime regimes.

Contribution

The paper introduces a multi-frequency extension of the M1 radiation treatment in Cosmos++, including Doppler and gravitational shifts, with an implicit scheme for stability in stiff regimes.

Findings

Validated against a wide range of test problems.

Successfully handled optically thin and thick regimes.

Demonstrated stability in strongly curved spacetimes.

Abstract

We report on recent upgrades to our general relativistic radiation-magnetohydrodynamics code, Cosmos++, which expands the two-moment, $\bf{M}_1$, radiation treatment from grey to multi-frequency transport, including Doppler and gravitational frequency shifts. The solver accommodates either photon (Bose-Einstein) or neutrino (Fermi-Dirac) statistical distribution functions with absorption, emission, and elastic scattering processes. An implicit scheme is implemented to simultaneously solve the primitive inversion problem together with the radiation-matter coupling source terms, providing stability over a broad range of opacities and optical depths where the interactions terms can be stiff. We discuss our formulations and numerical methods, and validate our methods against a wide variety of test problems spanning optically thin to thick regimes in flat, weakly curved, and strongly curved spacetimes.