ASTROPHYSICS

The recent observations of gravitational waves (GWs) marked a breakthrough and inaugurated the field of GW astronomy. To extract information from a detection, the measured signal needs to be cross-correlated with a template family. However, due to the nonlinearity of Einstein’s equations, numerical simulations have to be used to study systems with gravitational fields strong enough to emit GWs. This project focused on the simulation of systems consisting of two neutron stars and investigated the effect of the mass ratio and the influence of the spin of the individual stars.

The recent observations of the gravitational wave (GW) signals produced by the coalescence of binary black holes has inaugurated the field of GW astronomy. Because of an increasing sensitivity of GW interferometers, several GWs are expected to be detected within the next years. Despite binary black hole systems, systems consisting of two neutron stars are one of the most promising sources.

To extract information from a detection, the measured signal is cross-correlated with a template family to obtain a best match. Thus, the theoretical modeling of the binary coalescence process is of primary importance to relate the source properties to the observed GW signal. But in contrast to black hole binaries a detection of binary neutron star systems allows also to understand the behaviour of material at supranuclear densities and therefore puts constraints on the unknown neutron star equation of state (EOS). To determine this part of the EOS is one of the large unsolved problems in modern astrophysics.

Although analytical models exist for compact binary systems when the constituents are well separated, those models fail for the last orbits before merger and numerical simulation is needed. Due to the nonlinearity of the equations and the large separation of length scales, these simulations are computationally demanding and need to be run on large supercomputers. For the simulations 36 million core hours of computing time were used within the project Binary Neutron Star Mergers on the LRZ HPC system SuperMUC. To continue their research work, the scientists were granted additional 45 million core hours of compute time for the following year. The project focused on two aspects: the effect of the mass ratio and the influence of the spin of the individual stars.

The simulations revealed that higher mass ratio setups eject more material. This material, which leaves the system, will emit electromagnetic waves and, therefore, brighter electromagnetic signals are expected for unequal mass binary neutron star mergers. The correlation between the GW signal and electromagnetic observables is of big importance for the field of multimessenger astronomy and numerical simulations have to be used once a GW of a binary neutron star configuration is measured.

The rotation of the individual neutron stars instead has a large influence on the GW signal itself, since it effects the inspiral dynamics several orbits before the merger. Thus, neglecting spin effects, which was done in most binary neutron stars simulations in the past, will lead to biases if one tries to extract information from an upcoming GW detection.