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

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.

23 pages, 20 figures, 2 tables, submitted to ApJ, high resolution version of the paper available at http://hermes.as.arizona.edu/~luc/ms.pdf

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