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General Relativistic Moving-Mesh Simulations of Neutron-Star Mergers

Periodic Reporting for period 1 - GreatMoves (General Relativistic Moving-Mesh Simulations of Neutron-Star Mergers)

Reporting period: 2018-07-01 to 2019-12-31

The focus of the GreatMoves project are neutron star mergers, i.e. compact stars made of high-density matter, which orbit around each other and eventually collide. This merging process produces a strong signal of gravitational waves, which in fact have been observed from such an event for the first time in 2017. Importantly, the gravitational waves carry information about the dynamics of the merger and thus the properties of high-density neutron star matter, which are not fully understood yet. Matter becoming unbound from the merger site expands with high velocities and forms heavy elements like gold and uranium through the so-called rapid neutron-capture process. This process heats the ejecta, which produces an electromagnetic emission. This signal has also been detected in 2017 and encodes details of the underlying mass ejection and nucleosynthesis. GreatMoves has the ambition to link the different observables of neutron star mergers to fundamental questions of physics. This concerns in particular the properties of high-density matter and the details of the element formation. To this end GreatMoves employs and develops computer codes to model the merging process and its observables. This represents a crucial step to fully exploit current and future measurements of neutron star mergers.
Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far (For the final period please include an overview of the results and their exploitation and dissemination:

Since the start of the project we have work on different aspects of the research subject. Some achievements are rather technical developments other concern new findings towards the project's objective of linking merger observables to fundamental physics questions.

We successfully implemented and test a general relativistic hydrodynamics solver in an existing moving mesh hydrodynamics tool. The new code has been employed to simulate isolated neutron stars. The code has been coupled with a solver for the Einstein equations to compute the gravitational field (adopting the conformal flatness condition). This implementation has also been benchmarked by test calculations.

We have calculated gravitational waveforms of neutron star mergers and worked within a collaboration on gravitational wave data analysis. By this we quantified the prospects of extracting equation of state information from the postmerger phase.

We have worked on understanding the equation of state dependence of the gravitational wave signal of neutron star mergers. To this end we simulated a large number of binary systems with different equation of state models. Towards the main objectives of our project we identified an unambiguous signature of a phase transtion to deconfined quark matter in neutron star mergers. This work demonstrated the importance to measure postmerger gravitational wave emission.
"We have implemented a general relativistic hydrodynamics solver with a dynamical space time in a moving-mesh hydrodynamics code to simulate neutron stars. This is the first code of this type world-wide. While we are not yet in the position to calculate mergers of neutron stars, the implementation of this novel scheme promises significant improvements for the modeling of highly dynamical fluid flow problems in relativistic astrophysics in the future.

By elaborating on gravitational wave data analysis methods for the postmerger phase we have identified the currently most promising method to actually extract information from a future measurement. As also demonstrated by our work on the hadron-quark phase transition, measuring postmerger gravitational wave emission is highly important to fully exploit the scientific value of future detections.

We have determined the strongest and cleanest signature of quark matter in neutron star mergers so far. By this work we show the unique opportunities to learn about the properties of high-density matter by detecting gravitational waves from the pre- and postmerger stage of neutron star coalescences. These results appeared in Physical Review Letters and our publication has been selected as ""Editor's Suggestion""."