Thermodynamics conditions of matter in the neutrino decoupling region during neutron star mergers

In 1908.04952 we analyse general relativistic merger simulations employing microphysical equations of state and approximate neutrino transport to investigate the thermodynamics conditions at which weak and thermal equilibrium freezes out (equilibrium surfaces), as well as conditions at which thetransition between diffusion and free-streaming regime occurs (diffusion surfaces). We find that the rest mass density and the neutrino energy are the most relevant quantities in determining the location of the decoupling surfaces. For example, for mean neutrinos energies, diffusion surfaces are located around 10^11 g ccm for all neutrino species (electron neutrinos and antineutrinos and mu-tau neutrinos). Equilibrium surfaces of electron neutrinos and antineutrinos are close to diffusion surfaces while for heavy flavor neutrinos they are significantly deeper (10^12 g ccm). Neutrinos streaming at infinity with different energies come from very different regions of the remnant: while low energy neutrinos (∼3 MeV) decouple at ρ∼10^13 g ccm, T∼10MeV and Ye~0.1 close to cold weak equilibrium, high energy neutrinos (∼50 MeV) decouple from the disk at ρ∼10^9 g ccm, T∼2 MeV and Ye~0.25. The presence of a massive NS or of a BH in the remnant influencesthe neutrino thermalization process. While in the former case neutrinos of all relevant energies thermalize and diffuse from the innermost part of the remnant, the lower maximum rest mass density in the BH-disk system does not allow a relevant portion of the neutrino spectrum to thermalize and diffuse. We conclude that the accurate modelling of neutrinos and of their emitted spectrum in BNS mergers requires the consistent inclusion of neutrino-matter interactions for a broad range of nuclear conditions, characterized by more than four order of magnitudes in rest mass densities, one order of magnitude in temperatures, and a factor of a few in electron fraction.

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