# The Event Horizon General Relativistic Magnetohydrodynamic Code Comparison Project

## TL;DR

This paper compares nine general relativistic magnetohydrodynamics (GRMHD) codes on a common accretion flow problem, demonstrating their maturity, consistency, and improved agreement with higher resolution.

## Contribution

It provides a comprehensive comparison of nine GRMHD codes, establishing their reliability and consistency for modeling astrophysical phenomena near compact objects.

## Key findings

- Code agreement improves with resolution
- Community of GRMHD codes is mature and capable
- Codes produce consistent results on test problems

## Abstract

Recent developments in compact object astrophysics, especially the discovery of merging neutron stars by LIGO, the imaging of the black hole in M87 by the Event Horizon Telescope (EHT) and high precision astrometry of the Galactic Center at close to the event horizon scale by the GRAVITY experiment motivate the development of numerical source models that solve the equations of general relativistic magnetohydrodynamics (GRMHD). Here we compare GRMHD solutions for the evolution of a magnetized accretion flow where turbulence is promoted by the magnetorotational instability from a set of nine GRMHD codes: Athena++, BHAC, Cosmos++, ECHO, H-AMR, iharm3D, HARM-Noble, IllinoisGRMHD and KORAL. Agreement between the codes improves as resolution increases, as measured by a consistently applied, specially developed set of code performance metrics. We conclude that the community of GRMHD codes is mature, capable, and consistent on these test problems.