The impact of numerical and algorithmic choices on the physical outcomes of GRMHD simulations of binary neutron star mergers*

General-relativistic magnetohydrodynamics (GRMHD) simulations are crucial to understanding the physics of binary neutron star (BNS) mergers. However, some of the physically relevant observables in BNS mergers simulations — such as the gravitational wave signal in the inspiral phase and the lifetime of the hypermassive neutron star remnant — seem to depend on apparently irrelevant numerical and algorithmic choices.

In this talk, I will briefly introduce two GRMHD codes, namely Spritz and IllinoisGRMHD, and I will outline the relevant differences in the way they solve the GRMHD equations and in some numerical choices they adopt. Then I will present a set of BNS merger simulations, each performed with both codes, and I will discuss the differences in the physical outcomes of the runs depending on which of the two codes is chosen to perform them. Finally, I will try to link the differences in the physical observables coming from the simulations to some differences in numerical and/or algorithmic choices of each of the two codes.