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

In this work we investigate the thermodynamics conditions at which neutrinos decouple from matter in neutron star merger remnants by post-processing results of merger simulations. We find that the matter density and the neutrino energies are the most relevant quantities in determining the decoupling surface location. For mean energy neutrinos (\(\sim \) 9, 15 and 24 MeV for \(\nu _e\), \(\bar{\nu }_e\) and \(\nu _{\mu ,\tau }\), respectively) the transition between diffusion and free-streaming conditions occurs around \(10^{11}\mathrm{g}~\mathrm{cm}^{-3}\) for all neutrino species. Weak and thermal equilibrium freeze-out occurs deeper (several \(10^{12}\mathrm{g}~\mathrm{cm}^{-3}\)) for heavy-flavor neutrinos than for \(\bar{\nu }_e\) and \(\nu _e\) (\(\gtrsim 10^{11}\mathrm{g}~\mathrm{cm}^{-3}\)). Decoupling temperatures are broadly in agreement with the average neutrino energies, with softer equations of state characterized by \(\sim \)1 MeV larger decoupling temperatures. Neutrinos streaming at infinity with different energies come from different remnant parts. While low-energy neutrinos (\( \sim 3~\mathrm{MeV}\)) decouple at \( \rho \sim 10^{13}\mathrm{g}~\mathrm{cm}^{-3}\), \(T \sim 10~\mathrm{MeV}\) and \(Y_e \lesssim 0.1\) close to weak equilibrium, high-energy ones (\( \sim 50~\mathrm{MeV}\)) decouple from the disk at \(\rho \sim 10^{9}\mathrm{g}~\mathrm{cm}^{-3}\), \(T \sim 2~\mathrm{MeV}\) and \(Y_e \gtrsim 0.25\). The presence of a massive NS or a BH influences the neutrino thermalization. While in the former case decoupling surfaces are present for all relevant energies, the lower maximum density (\(\lesssim 10^{12}\mathrm{g}~\mathrm{cm}^{-3}\)) in BH-torus systems does not allow softer neutrinos to thermalize and diffuse.