# A radiative transfer module for relativistic magnetohydrodynamics in the PLUTO code

## TL;DR

This paper introduces a new module within the PLUTO code for solving relativistic radiation magnetohydrodynamics equations, capable of handling complex astrophysical phenomena with various coordinate systems and adaptive grids.

## Contribution

It implements a novel numerical scheme with an implicit-explicit approach and a new HLLC solver for radiation transport, enhancing the capabilities of the PLUTO code.

## Key findings

- Successfully demonstrated through numerical benchmarks
- Handles both free-streaming and diffusion limits effectively
- Supports multidimensional and adaptive grid computations

## Abstract

We present a numerical implementation for the solution of the relativistic radiation hydrodynamics and magnetohydrodynamics equations, designed as an independent module within the freely available code PLUTO. The radiation transfer equations are solved under the grey approximation and imposing the M1 closure, which allows the radiation transport to be handled in both the free-streaming and diffusion limits. Equations are integrated following an implicit-explicit scheme, where radiation-matter interaction terms are integrated implicitly, whereas transport and all of the remaining source terms are solved explicitly by means of the same Godunov-type solvers included in PLUTO. Among these, we introduce a new Harten-van Leer-contact (HLLC) solver for optically thin radiation transport. The code is suitable for multidimensional computations in Cartesian, spherical and cylindrical coordinates, using either a single processor or parallel architectures. Adaptive grid computations are also made possible, by means of the CHOMBO library. The algorithm performance is demonstrated through a series of numerical benchmarks by investigating various different configurations with a particular emphasis on the behavior of the solutions in the free-streaming and diffusion limits.