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Gravitational Waves and Electromagnetic Counterparts from Generic Binary Neutron Star Systems

Periodic Reporting for period 1 - BNSmergers (Gravitational Waves and Electromagnetic Counterparts from Generic Binary Neutron Star Systems)

Reporting period: 2018-01-15 to 2020-01-14

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.
We derived the mathematical methods to simulate generic binary neutron star systems, simultaneously varying the masses, spins, eccentricity, and internal composition of the stars. We collected all numerical relativity simulations performed over the last years and made them publicly available. Our database is the largest set of binary neutron star simulations. More than 350 simulations have been uploaded, making use of more than 250 million CPU hours. Until now the webpage has been used by more than 1000 scientists from more than 100 countries.

To allow a proper interpretation of measured gravitational-wave signals, one needs to compare the gravitational wave detector data with theoretical predictions. These predictions need to be accurate, to allow a proper interpretation of the astrophysical sources but also need to be fast since millions of theoretical predictions have to be compared to the measurements.
For this reason, we derived an analytical, closed-form gravitational wave model that employs directly high-resolution and error-controlled numerical relativity data. The latter has been combined with analytical and semi-analytical expressions. This allowed us to build a waveform model that is valid from the low frequencies to the strong-field regime and up to the merger. This work provided for the first time simple, flexible, and accurate templates that were used directly in the data analysis of the first and second binary neutron star detections observed by LIGO and Virgo. The model is to date the standard waveform approximant within the LIGO Scientific Community for the interpretation of neutron star binaries. The continuous improvement of the model has been a key aspect of the research project. We also quantified to what extent uncertainties of the employed waveform models could lead to biases during the interpretation of measured data.

Due to the combined detection of gravitational waves and electromagnetic signals ejected from the first binary neutron star merger, the field of multimessenger astronomy has made a giant leap. We showed how the detected kilonova lightcurve can be used to infer source properties and employed different state-of-the-art kilonova models to derive bounds for the equation of state of supranuclear densities. We also showed that kilonovae on their own can be used to determine the expansion rate of the Universe.

We also investigated how our derived tools could be used to measure dark matter in the Universe or if the observed electromagnetic signals could also be produced during the collapse of rotating neutron stars.
We presented simulations in areas of the binary neutron star parameter space which has not been accessible by any other numerical relativity group.
We showed that the closed-form waveform approximant derived from our simulations can be employed for the interpretation of real gravitational wave signals.
This model is the standard tool for the analysis of gravitational-wave signals of binary neutron stars and to-date the only model describing precessing binary neutron stars.
We performed the first multi-messenger analysis in which information from gravitational waves, the optical kilonova, and the afterglow of the short gamma-ray-burst have been combined to allow a Bayesian analysis of the source.
This study and similar work proved that multi-messenger constraints can be important to understand the behavior of matter at supranuclear densities and provide an independent way to measure the expansion rate of the universe.
GWs emitted from a BNS merger. [Image used for cover page to announce first BNS detection.]