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 eveloped novel, innovative codes or code modules and adapted existing numerical tools for the particular requirements to describe observables of neutron star mergers.
