GRHydro: a new open-source general-relativistic magnetohydrodynamics code for the Einstein toolkit

Abstract

We present the new general-relativistic magnetohydrodynamics (GRMHD)\ncapabilities of the Einstein Toolkit, an open-source community-driven numerical\nrelativity and computational relativistic astrophysics code. The GRMHD\nextension of the Toolkit builds upon previous releases and implements the\nevolution of relativistic magnetised fluids in the ideal MHD limit in fully\ndynamical spacetimes using the same shock-capturing techniques previously\napplied to hydrodynamical evolution. In order to maintain the divergence-free\ncharacter of the magnetic field, the code implements both hyperbolic divergence\ncleaning and constrained transport schemes. We present test results for a\nnumber of MHD tests in Minkowski and curved spacetimes. Minkowski tests include\naligned and oblique planar shocks, cylindrical explosions, magnetic rotors,\nAlfv\\'en waves and advected loops, as well as a set of tests designed to study\nthe response of the divergence cleaning scheme to numerically generated\nmonopoles. We study the code's performance in curved spacetimes with spherical\naccretion onto a black hole on a fixed background spacetime and in fully\ndynamical spacetimes by evolutions of a magnetised polytropic neutron star and\nof the collapse of a magnetised stellar core. Our results agree well with exact\nsolutions where these are available and we demonstrate convergence. All code\nand input files used to generate the results are available on\nhttp://einsteintoolkit.org. This makes our work fully reproducible and provides\nnew users with an introduction to applications of the code.\n