Thermodynamics conditions of matter in neutron star mergers

In 1903.07898 we explore the properties of extreme matter along the general relativistic dynamics of neutron star mergers. We consider 3 microphysical equations of state and numerical relativity simulations including an approximate neutrino transport scheme and construct time-dependent histograms of the mass on a 3D hypersurfaces as distributed in density, temperature and electron fraction bins (see figure below). For example, the matter in a merger ending in a long-lived neutron star increases the initial maximum density up to ∼3-4ρ0 (ρ0 is the nuclear saturation density), and the multiple centrifugal bounces heat the initially cold matter to several tens of MeV. Streams of hot matter with initial densities ∼2ρ0 move outwards and cool due to decompression and neutrino emission. The remnant reaches temperatures up to 50 MeV, but the highest temperatures are confined in an approximately spherical annulus at ∼ρ0. Such temperatures favour positron-neutron capture and lead to a neutrino emission dominated by electron antineutrinos. Neutrinos are trapped at the highest densities of the remnant core which is less affected by weak interaction and only at longer timescales. A similar dynamics is found if the remnant collapses to black hole. The main difference bewteen the two case is found in the remnant disk. Both discs are neutron rich and not isentropic, but they differ in size, entropy and lepton fraction depending on the nature of the central object. In presence of a black hole, disks are smaller and mostly transparent to neutrinos; in presence of a massive neutron star, they are more massive, geometrically and optically thick.

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