High eccentricities and rapid spins in neutron star binaries

This dissertation presents a comprehensive numerical investigation of the matter dynamics, gravitational wave emission, nucleosynthetic yields, and potential electromagnetic afterglow properties of numerous extreme neutron star binary mergers, incorporating varying equations of state as well as orbital and stellar properties. In particular, the thesis explores the potential of post-merger observational signatures associated with gravitational wave events to reveal the pre-merger properties of such systems. In addition to the study of highly eccentric binary orbits and their impact on the merger dynamics of binary neutron stars as well as on the post-merger properties, a large part of this thesis focuses on the effects of rapidly spinning neutron star binary companions. In particular, the intriguing case of GW190814 is examined, where the secondary object is proposed to be a rapidly spinning neutron star that may have collapsed to a black hole before merger, yielding new estimates for the spin of the secondary object and providing a strict lower bound on the maximum mass of non-rotating neutron stars. To facilitate further research in this area, an open-source spectral elliptic solver infrastructure will be established, capable of constructing binary black holes, binary neutron stars, and mixed binaries under a wide range of stellar and binary parameters. The effect of high stellar spins on mass ejection in binary neutron star mergers is then investigated, showing that high spins aligned with the orbital angular momentum can potentially suppress the dynamical component of the ejected mass. In high-mass binary neutron star mergers, further investigation reveals that rapid dynamical mass ejection resulting from the spin-up of the primary star during the merger may serve to distinguish neutron stars from black holes. In addition, a close examination of high-mass, high-spin binary neutron star mergers shows that these can significantly affect remnant lifetimes, ejecta dynamics, remnant disk masses, and kilonova properties, potentially helping to distinguish between low- and high-spin priors in the detection of gravitational waves from such events. The thesis culminates with a comprehensive analysis of the configurations that are equivalent under the range of possible stellar parameters of a high-spin prior gravitational wave event at a fixed chirp mass, thus providing a deeper understanding of the potential post-merger observables for binary neutron star mergers and underscoring the significant impact of varying stellar parameters on these observables.