Neutrino signatures and the neutrino-driven wind in Binary Neutron Star Mergers

TL;DR

This study uses advanced simulations to analyze neutrino emissions and wind dynamics in binary neutron star mergers, revealing polar neutrino luminosities, wind formation, and potential implications for gamma-ray burst production.

Contribution

It introduces a multi-angle neutrino transport formalism and detailed analysis of neutrino signatures and wind properties in BNS mergers, enhancing understanding of post-merger phenomena.

Findings

Polar-enhanced neutrino luminosities observed

Formation of a thermally-driven bipolar wind

Estimated neutrino annihilation rates decrease over time

Abstract

We present VULCAN/2D multi-group flux-limited-diffusion radiation hydrodynamics simulations of binary neutron star (BNS) mergers, using the Shen equation of state, covering ~100 ms, and starting from azimuthal-averaged 2D slices obtained from 3D SPH simulations of Rosswog & Price for 1.4 Msun (baryonic) neutron stars with no initial spins, co-rotating spins, and counter-rotating spins. Snapshots are post-processed at 10 ms intervals with a multi-angle neutrino-transport solver. We find polar-enhanced neutrino luminosities, dominated by $\bar{\nu}_e$ and ``$\nu_\mu$'' neutrinos at peak, although $\nu_e$ emission may be stronger at late times. We obtain typical peak neutrino energies for $\nu_e$, $\bar{\nu}_e$, and ``$\nu_\mu$'' of ~12, ~16, and ~22 MeV. The super-massive neutron star (SMNS) formed from the merger has a cooling timescale of ~1 s. Charge-current neutrino reactions lead to the formation of a thermally-driven bipolar wind with <$\dot{M}$> ~10$^{-3}$ Msun/s, baryon-loading the polar regions, and preventing any production of a GRB prior to black-hole formation. The large budget of rotational free energy suggests magneto-rotational effects could produce a much greater polar mass loss. We estimate that ~10$^{-4}$ Msun of material with electron fraction in the range 0.1-0.2 become unbound during this SMNS phase as a result of neutrino heating. We present a new formalism to compute the $\nu_i\bar{\nu}_i$ annihilation rate based on moments of the neutrino specific intensity computed with our multi-angle solver. Cumulative annihilation rates, which decay as $t^{-1.8}$, decrease over our 100 ms window from a few 10$^{50}$ to ~10$^{49}$ erg/s, equivalent to a few 10$^{54}$ to ~10$^{53}$ $e^-e^+$ pairs per second.