With the first direct detections of gravitational waves emitted by the coalescence of two black holes, a new era in astronomy has been inaugurated. Shortly afterwards followed the breakthrough observation of gravitational waves and electromagnetic signals from the same source, the coalescence and merger of binary neutron stars. Due to the increasing sensitivity of the gravitational wave detectors in upcoming years, we expect to detect several gravitational wave sources possibly in combination with electromagnetic signals. Large velocities and strong gravitational fields require one to solve Einstein’s Field Equations numerically to study the last stages of the binary coalescence. Indeed, numerical relativity is the fundamental tool to study gravitational waves from systems in the strong-field regime.
The research project aims to provide a better understanding of the binary neutron star merger process, with a view on the detection of gravitational waves and electromagnetic signals from generic binary configurations. We simulate a large number of binary neutron star systems with various masses, mass ratios, equations of states, spins, and eccentricities. These simulations allow one to model the emitted gravitational wave signal and to approximate the corresponding electromagnetic transient of binary neutron star mergers.
Overall, the project combines three main objectives.
Firstly, the coverage of the binary neutron star parameter space with numerical simulations. One needs numerical simulations in full general relativity to give accurate theoretical predictions for the properties of merging binary neutron stars. Due to the strong nonlinearity of the equations and the large separation of length scales, numerical relativity simulations are computationally demanding, and need to be run on large supercomputers. Nevertheless, we simulate a large number of configuration for various binary parameters for a reasonable coverage of the binary neutron star parameter space to be prepared for observations of generic binary systems. To better support the new field of gravitational-wave astronomy, we make our waveforms publicly available such that scientists without the necessary tools and computational resources can use our results for their study.
Secondly, the development of new and the upgrade of existing gravitational wave models.
To extract information from a gravitational wave detection, the gravitational wave signal is cross-correlated with a template family to obtain 'the best match'. Consequently, the theoretical modeling of the binary coalescence process is of primary importance to relate the source properties to the observed signal. In contrast to black hole binaries, a detection of binary neutron stars allows one to understand the behavior of the material at supranuclear densities. Therefore, we can place constraints on the unknown neutron star equation of state.
Thirdly, modeling the electromagnetic transient of binary neutron star mergers and enabling a multi-messenger interpretation.
Numerical simulations provide estimates of the amount of neutron-rich ejected material which becomes unbound from the system. This material is the seed for the creation of heavy elements, e.g. Gold and Platinum, in the Universe and creates a typical electromagnetic counterpart known as 'kilonova'. The luminosity, duration, and temperature of a kilonova depend on the mass, velocity, composition, and geometry of the ejected material and, therefore, contain valuable information about the properties of the progenitor system.
The simultaneous interpretation of gravitational wave signals and electromagnetic observations of binary neutron star mergers is a central aspect in the rising field of multi-messenger astronomy and strongly supported by the research project.
