NEUTRINO SIGNATURES AND THE NEUTRINO-DRIVEN WIND IN BINARY NEUTRON STAR MERGERS

Abstract

We present VULCAN/2D multigroup flux-limited-diffusion radiation-hydrodynamics simulations of binary neutron star mergers, using the Shen equation of state, covering ≳ 100 ms, and starting from azimuthal-averaged two-dimensional slices obtained from three-dimensional smooth-particle-hydrodynamics simulations of Rosswog & Price for 1.4 M ☉ (baryonic) neutron stars with no initial spins, co-rotating spins, or counter-rotating spins. Snapshots are post-processed at 10 ms intervals with a multiangle neutrino-transport solver. We find polar-enhanced neutrino luminosities, dominated by and "ν μ " neutrinos at the peak, although ν e emission may be stronger at late times. We obtain typical peak neutrino energies for ν e , , and "ν μ " of ∼ 12, ∼ 16, and ∼ 22 MeV, respectively. The supermassive 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 10 −3 M ☉ s −1 and baryon-loading in the polar regions, preventing any production of a γ-ray burst prior to black hole formation. The large budget of rotational free energy suggests that magneto-rotational effects could produce a much-greater polar mass loss. We estimate that ≲ 10 −4 M ☉ of material with an electron fraction in the range 0.1–0.2 becomes unbound during this SMNS phase as a result of neutrino heating. We present a new formalism to compute the annihilation rate based on moments of the neutrino-specific intensity computed with our multiangle 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 −1 , equivalent to a few × 10 54 to ∼ 10 53 e − e + pairs per second.