Importing Binary Neutron Star initial data from SGRID into the Einstein Toolkit

We are investigating Binary Neutron Star mergers by performing numerical simulations using the Einstein Toolkit (ET) framework - Cactus.

A simulation of this kind is a sophisticated mathematical problem. It involves solving many coupled second order, non-linear, partial differential equations coming from Einstein's equations together with hydrodynamics. To prepare a numerical setup for a simulation of this kind, we encounter a series of challenges. An essential part of such a setup is the problem of creating Initial Data (ID).

In order to even start such simulations, realistic ID are needed that accurately encode the state of both stars in orbit prior to merger. We use the SGRID code, which has been developed by the numerical relativity group at FAU. It allows us to prepare ID for the binary system where the stars are characterized by arbitrary masses, spins and orbital eccentricities together with an equation of state. Different scientific group have already developed different tools to produce ID. So far, the responsibility to prepare it in a compatible and readable form has largely remained on the user side. To simplify this problem, we have developed a Cactus thorn called DNSdata, that can read SGRID ID into the ET, so that the ET can then perform the subsequent evolution.

We will discuss the advantages of the SGRID code for ID creation, the details of the DNSdata thorn, and the results of a first simulation.