This year gravitational waves were detected for the first time, from a pair of merging black holes. An understanding of neutron star mergers will be crucial for the interpretation of their gravitational wave properties.
Funded by the EU, the objective of the project EOSDNSM (Equation of state dependence of neutron star mergers) was to investigate neutron star mergers using numerical simulations. An equation of state in physics is a mathematical relationship that is always true between the important state variables like pressure and density. The studies had two main goals: a deeper understanding of the gravitational wave emission of these events, and an investigation of the role of mergers in the production of heavy elements formed by the rapid neutron-capture nuclear process.
The project team performed a large set of neutron star merger simulations with many different temperature-dependent equations of state and with systematically varied binary masses. The gravitational wave spectra were analysed and empirical relations for different features of the spectrum were derived.
Oscillation modes and dynamical features of the merger remnant were identified and linked to features of the gravitational wave signal. The simulated gravitational wave signals were used in gravitational wave data analysis studies to explore their detectability and expected detection rate.
The enlarged set of merger simulations has confirmed previous findings of a tight relation between the dominant gravitational wave oscillation frequency and neutron star radii. Inferring neutron star radii from gravitational wave observations rests on the precision of this relation. Project results will therefore facilitate an accurate measurement of neutron star radii. A determination of the maximum mass of neutron stars, also supported by the project results, will have an impact on high-density matter physics.
EOSDNSM results have provided an understanding regarding which of the different mechanisms are particularly strong for a given binary system and equation of state. This led to a unified classification scheme of the post-merger dynamics and gravitational wave emission.
The work will help improve understanding of the relevant nuclear reactions during mergers and the underlying theoretical nuclear models. The finding of a clear equation of state dependence of the ejecta properties is relevant for chemical evolution models as well as for the interpretation of future observations of electromagnetic counterparts.