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General Relativistic Moving-Mesh Simulations of Neutron-Star Mergers

Periodic Reporting for period 3 - GreatMoves (General Relativistic Moving-Mesh Simulations of Neutron-Star Mergers)

Reporting period: 2021-07-01 to 2022-12-31

In our project we aim at the theoretical description of neutron star merger events mostly with computer codes. Since neutron stars are the densest objects in the universe, their collision results in a variety of different observable phenomena which are linked to very fundamental physical processes. This implies that the observation and interpretation of these events can inform about unknown physics. The very first unambiguous detection of a neutron star merger has succeeded just before the start of our project and has impressively confirmed this picture and has already provided a wealth of new information about the merger process and the underlying physics.

An important part of the project focuses on the postmerger object and its gravitational wave emission. This is in particular linked to the so-called equation of state encoding properties of high-density matter and which thus affects the structure and the dynamics of the postmerger object. The ultimate goal is to extract information about unknown properties of high-density matter from observations of the postmerger remnant, i.e. its gravitational wave emission. This includes questions like those about the stiffness and constituents of nuclear matter or the occurrence of the hadron-quark phase transition, i.e. the transition of nuclear matter to a phase of deconfined quarks.

Another important aspect of neutron star mergers and our work concerns the outflows from these events. Because of the extreme conditions in the environment of neutron star mergers they are perfectly suited to synthesize heavy elements such as gold and platinum whose origin is not yet fully understood. To this end we connect detailed hydrodynamical models of the various mass ejection channels with calculations to predict the elemental distribution in the outflows. In this regard it is critical to take into account the influence of weak interactions on the initial composition of the ejecta because it sensitively affects the production of heavy elements.

The synthesis of heavy elements is connected with the release of energy in the form of radioactive decays which heat the ejecta. The heat being deposited in the ejecta leads to emission of electromagnetic radiation in the optical and infrared wavebands. The emission may be sufficiently powerful to be observable by telescopes if they identify the electromagnetic counterpart of the merger before its luminosity significantly decreases after several days. In fact such emission has been observed for the first neutron star merger and proven that one can extract information about mass ejection and element formation from the properties of these electromagnetic transients. Along these lines our project develops models to accurately and reliably interpret such observations to elucidate the process of element formation in the universe.

Towards these goals our action develops novel, innovative codes or code modules and adapts existing numerical tools for the particular requirements to describe observables of neutron star mergers.
From the beginning of the project we have worked on different technical developments and several scientific studies. We summarize the main activities.

We have studied the impact of a phase transition on the observables of neutron star mergers and identified an unambiguous signature which indicates the occurrence of deconfined quark matter in these events.

We conducted a large systematic study of the merger outcome to understand the conditions for black hole formation. This has lead to improved equation of state constraints from the first detection of a neutron star merger GW170817.

We worked on GW asteroseismology relations for neutron star mergers. We finalized a comprehensive study and linked the GW emission in mergers to the properties of static neutron stars by using results from an existing perturbative code.

We have successfully implemented a relativistic hydrodynamics solver in a moving-mesh code and coupled the tool with a solver for the Einstein field equations.

We are about to finalize a comprehensive study of secular ejecta from neutron star mergers and of the impact of weak interactions through a truncated moment scheme for radiation transport.

Very first steps were taken towards radiative transfer calculations for the electromagnetic emission of neutron star mergers.
In several respects our results represent achievements beyond state of the art. This concerns for instance the in-depth investigation of phase transitions in neutron star mergers. Our work on asteroseismology of neutron star merger remnants and the comprehensive study of black hole formation in mergers significantly extends previous knowledge. Similarly, our collaborative work on developing appropriate methods for the detection of the postmerger gravitational wave emission is at the forefront of research. A relativistic moving-mesh hydrodynamics code with a dynamical space time evolution is to our knowledge world-wide unique.

In the remaining time of the project we plan to further continue and exploit the technical developments along the outlined scientific goals. This concerns in particular also studies of the mass ejection from mergers and the associated nucleosynthesis. We will improve the modelling of the electromagnetic emission of neutron star mergers employing radiative transfer models.
Simulation of a neutron-star merger