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Binary Neutron Star Merger Simulations

Principal Investigators: Bernd Brügmann, Friedrich-Schiller-University, Jena, and Tim Dietrich, Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Potsdam-Golm (Germany)
HPC Platform: SuperMUC of LRZ

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.

Binary Neutron Star MergersFigure 1: 3D rendering of the gravitational waves emitted from a binary neutron star system at merger. The central region (density) is stretched by a factor of ~5 for better visibility of the tidal deformation.
Copyright: AEI Potsdam-Golm

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.

Binary Neutron Star MergersFigure 2: Gravitational wave signal (upper panel) and snapshots of the density profile (lower panels). Left: early inspiral, middle: merger, where a large amount of material is ejected (blue), right: merger remnant, surrounded by an accretion disk.
Copyright: AEI Potsdam-Golm

In total, studying a large portion of the binary neutron star parameter space allows to create templates, crosscheck semi-analytical models, and to estimate the electromagnetic counterparts visible during and after the collision of the two neutron stars. The work strongly supports the field of GW and multimessenger astronomy.

Research Team:

Sebastiano Bernuzzi1, Bernd Brügmann2 (PI), Tim Dietrich3 (PI), Maximiliano Ujevic Tonino4, Wolfgang Tichy5

1 DiFeST, University of Parma, and INFN Parma (Italy)
2 Friedrich-Schiller-University, Jena (Germany)
3 Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Potsdam-Golm (Germany)
4 Centro de Ciencias Naturais e Humanas, Universidade Federal do ABC, Santo Andre, Sao Paulo (Brazil)
5 Department of Physics, Florida Atlantic University, Boca Raton, FL (USA)

Publications:

Dietrich, Tim and Ujevic, Maximiliano. Modeling dynamical ejecta from binary neutron star mergers and implications for electromagnetic counterparts. ArXiv: 1612.03665, 2016

Dietrich, Tim and Bernuzzi, Sebastiano and Ujevic, Maximiliano and Tichy, Wolfgang. Gravitational waves and mass ejecta from binary neutron star mergers: Effect of the stars' rotation. arXiv: 1611.07367, 2016

Dietrich, Tim and Ujevic, Maximiliano and Tichy, Wolfgang and Bernuzzi, Sebastiano and Bruegmann, Bernd. Gravitational waves and mass ejecta from binary neutron star mergers: Effect of the mass-ratio. arXiv: 1607.06636, 2016

Bernuzzi, Sebastiano and Dietrich, Tim. Gravitational waveforms from binary neutron star mergers with high-order weighted-essentially-nonoscillatory schemes in numerical relativity. Phys. Rev. D94 064062, 2016

Scientific Contact:

Tim Dietrich
Max Planck Institute for Gravitational Physics (Albert Einstein Institute)
Science Park Potsdam-Golm
Am Mühlenberg 1, D-14476 Potsdam-Golm (Germany)
e-mail: tim.dietrich[at]aei.mpg.de

https://www.aei.mpg.de

December 2016

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