Abstract
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.
Highlights
The merger of two neutron stars (NSs) results in the emission of a burst of gravitational waves (GWs), in the formation of a massive remnant and in the ejection of matter in the interstellar medium; see [1,2,3] for some recent reviews
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
In this paper we have studied the thermodynamical properties of matter in the neutrino decoupling region of binary neutron star mergers remnants
Summary
The merger of two neutron stars (NSs) results in the emission of a burst of gravitational waves (GWs), in the formation of a massive remnant (possibly collapsing to a black hole, BH) and in the ejection of matter in the interstellar medium; see [1,2,3] for some recent reviews. The properties of the disk, as well as the timescale for BH collapse of the central object (if gravitationally unstable), depend mainly on the NS masses and on the still uncertain nuclear equation of state (EOS); see e.g. Due to its high neutron content, the ejecta expelled in the interstellar medium can immediately synthesize heavy elements via the so-called rapid neutron capture process (r -process) nucleosynthesis [11,12,13]. The merger remnant has been long thought to be the central engine of short-hard gamma-ray bursts, one of the most energetic and elusive extragalactic events detected up to cosmological distances [17,18]
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