An Extension of the Athena++ Code Framework for Radiation-Magnetohydrodynamics in General Relativity Using a Finite-Solid-Angle Discretization

TL;DR

This paper extends the Athena++ framework to include radiation in general-relativistic magnetohydrodynamics, enabling detailed simulations of black hole accretion with improved accuracy and computational efficiency.

Contribution

The authors develop a finite-solid-angle discretization method for radiation in GRMHD within Athena++, incorporating explicit transport and matter-radiation coupling, verified through tests and optimized for GPU computing.

Findings

Successful verification with test suites

Application to black hole accretion scenarios

Excellent scalability on high-performance clusters

Abstract

We extend the general-relativistic magnetohydrodynamics (GRMHD) capabilities of Athena++ to incorporate radiation. The intensity field in each finite-volume cell is discretized in angle, with explicit transport in both space and angle properly accounting for the effects of gravity on null geodesics, and with matter and radiation coupled in a locally implicit fashion. Here we describe the numerical procedure in detail, verifying its correctness with a suite of tests. Motivated in particular by black hole accretion in the high-accretion-rate, thin-disk regime, we demonstrate the application of the method to this problem. With excellent scaling on flagship computing clusters, the port of the algorithm to the GPU-enabled AthenaK code now allows the simulation of many previously intractable radiation-GRMHD systems.