Merging neutron stars are expected to produce hot, metastable remnants in rapid differential rotation, which subsequently cool and evolve into rigidly rotating neutron stars or collapse to black holes. Studying this metastable phase and its further evolution is essential for the prediction and interpretation of the electromagnetic, neutrino, and gravitational signals from such a merger. In this work, we model binary neutron star merger remnants and propose new rotation and thermal laws that describe postmerger remnants. Our framework is capable to reproduce quasiequilibrium configurations for generic equations of state, rotation and temperature profiles, including nonbarotropic ones. We demonstrate that our results are in agreement with numerical relativity simulations concerning bulk remnant properties like the mass, angular momentum, and the formation of a massive accretion disk. Because of the low computational cost for our axisymmetric code compared to full 3 + 1-dimensional simulations, we can perform an extensive exploration of the binary neutron star remnant parameter space studying several hundred thousand configurations for different equations of state.

Neutron stars provide an excellent laboratory for physics under the most extreme conditions. Up to now, models of axisymmetric, stationary, differentially rotating neutron stars were constructed under the strong assumption of barotropicity, where a one-to-one relation between all thermodynamic quantities exists. This implies that the specific angular momentum of a matter element depends only on its angular velocity. The physical conditions in the early stages of neutron stars, however, are determined by their violent birth processes, typically a supernova or in some cases the merger of two neutron stars, and detailed numerical models show that the resulting stars are by no means barotropic. Here, we construct models for stationary, differentially rotating, nonbarotropic neutron stars, where the equation of state and the specific angular momentum depend on more than one independent variable. We show that the potential formulation of the relativistic Euler equation can be extended to the nonbarotropic case, which, to the best of our knowledge, is a new result even for the Newtonian case. We implement the new method into the XNS code and construct equilibrium configurations for nonbarotropic equations of state. We scrutinize the resulting configurations by evolving them dynamically with the numerical relativity code BAM, thereby demonstrating that the new method indeed produces stationary, differentially rotating, nonbarotropic neutron star configurations.

Short gamma-ray bursts (sGRBs) show a large diversity in their properties. This suggests that the observed phenomenon can be caused by different 'central engines' or that the engine produces a variety of outcomes depending on its parameters, or possibly both. The most popular engine scenario, the merger of two neutron stars, has received support from the recent Fermi and INTEGRAL detection of a burst of gamma rays (GRB170817A) following the neutron star merger GW 170817, but at the moment, it is not clear how peculiar this event potentially was. Several sGRBs engine models involve the collapse of a supramassive neutron star that produces a black hole plus an accretion disc. We study this scenario for a variety of equations of states both via angular momentum considerations based on equilibrium models and via fully dynamical Numerical Relativity simulations. We obtain a broader range of disc forming configurations than earlier studies but we agree with the latter that none of these configurations is likely to produce a phenomenon that would be classified as an sGRB.

In a core-collapse supernova, a huge amount of energy is released in the Kelvin-Helmholtz phase subsequent to the explosion, when the proto-neutron star cools and deleptonizes as it loses neutrinos. Most of this energy is emitted through neutrinos, but a fraction of it can be released through gravitational waves. We model the evolution of a proto-neutron star in the Kelvin-Helmholtz phase using a general relativistic numerical code, and a recently proposed finite temperature, many-body equation of state; from this we consistently compute the diffusion coefficients driving the evolution. To include the many-body equation of state, we develop a new fitting formula for the high density baryon free energy at finite temperature and intermediate proton fraction. We estimate the emitted neutrino signal, assessing its detectability by present terrestrial detectors, and we determine the frequencies and damping times of the quasinormal modes which would characterize the gravitational wave signal emitted in this stage.