ASTROPHYSICS

Introduction

This is an ongoing project, with the goal to investigate the long-term evolution of a merging binary system of two neutron stars. The investigation conducted within this project is well aligned with the past research conducted by the Relastro group in Frankfurt [1]. It is motivated by the gravitational-wave detection GW170817 and its electromagnetic counterpart, the so-called kilonova. This kilonova signal is produced by the nuclear processes within the dense and neutron rich mass that is ejected during the merger. Since a lot of mass is ejected during the long-term postmerger evolution, it is crucial to investigate this part via state-of-the-art simulations in order to fully understand the observation.

Results and Methods
Such simulations require the solutions of a number of complex equations in order to account for all the physical processes that are relevant during the long-term evolution: general relativity for describing the strong gravitational fields, magneto-hydrodynamics for modeling the fluid making up the component neutron stars and the magnetic fields connected with that fluid, and radiative transport in order to account for neutrino radiation produced by the extreme conditions during the merger. The latter has just recently been added via the implementation of the new Frankfurt Radiation Code (FRAC) [2]. It has been used for the successful simulation of spherically symmetric accretion problems in order to test and verify the implementation in a realistic highenergy astrophysical scenario (see Fig. 1).

Currently, FRAC is being coupled to the general-relativistic magneto-hydrodynamics code FIL, which is part of the Einstein Toolkit, a framework for simulations in astrophysics. This coupling will result in a code with all the necessary physics modules in order to simulate the long-term evolution of a binary neutron star merger.

Such a merger is also related to the production of a short gamma-ray burst. The characteristics of such a jet have also been investigated within pn56bi supplementing the above described investigation of long-term simulations. For this purpose, magneto-hydrodynamical simulations of a black hole-torus system have been performed on SuperMUC-NG. These simulations shed light on how a jet like the one observed as counterpart to the gravitational-wave detection GW170817 is formed and how it propagates [3].

Jets are not only important for neutron stars, but also for active galactive nuclei – the centers of galaxies, where powerful jets emerge from supermassive black holes. In any case, a crucial role in producing jets is played by magnetic fields. To this scope a series of simulations was performed within pn56bi in order to study the generation of current sheets through instabilities triggered by the magnetic field [4].

Figure 2 shows the results of an exemplary simulation in terms of the magnetic field strength (left), the current (middle), and the magnetization (right). It was found that so-called plasmoids produced by these current sheets are a plausible explanation for the observed variability in the jets from active galactic nuclei.