Neutrino Trapping and Out-of-Equilibrium Effects in Binary Neutron-Star Merger Remnants.

We study out-of-thermodynamic-equilibrium effects in neutron-star mergers with 3D general-relativistic neutrino-radiation large-eddy simulations. During mergers, the cores of the neutron stars remain cold (T∼ a few MeV) and out of thermodynamic equilibrium with trapped neutrinos originating from the hot collisional interface between the stars. However, within ∼2 to 3 ms matter and neutrinos reach equilibrium everywhere in the remnant massive neutron star. Our results show that dissipative effects, such as bulk viscosity, if present, are only active for a short window of time after the merger.

Unveiling the Merger Structure of Black Hole Binaries in Generic Planar Orbits.

The precise modeling of binary black hole coalescences in generic planar orbits is a crucial step to disentangle dynamical and isolated binary formation channels through gravitational-wave observations. The merger regime of such coalescences exhibits a significantly higher complexity compared to the quasicircular case, and cannot be readily described through standard parametrizations in terms of eccentricity and anomaly. In the spirit of the effective one body formalism, we build on the study of the test-mass limit, and introduce a new modeling strategy to describe the general-relativistic dynamics of two-body systems in generic orbits. This is achieved through gauge-invariant combinations of the binary energy and angular momentum, such as a dynamical "impact parameter" at merger. These variables reveal simple "quasi-universal" structures of the pivotal merger parameters, allowing us to build an accurate analytical representation of generic (bounded and dynamically bounded) orbital configurations. We demonstrate the validity of these analytical relations using 311 numerical simulations of bounded noncircular binaries with progenitors from the RIT and SXS catalogs, together with a custom dataset of dynamical captures generated using the Einstein Toolkit, and test-mass data in bound orbits. Our modeling strategy lays the foundations of accurate and complete waveform models for systems in arbitrary orbits, bolstering observational explorations of dynamical formation scenarios and the discovery of new classes of gravitational wave sources.