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 twodimensional 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 polarenhanced neutrino luminosities, dominated byν e 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 ,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 Ṁ ∼ 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 ν iνi 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.

The role of weak interactions in dynamic ejecta from binary neutron star mergers

“…In fact, along the equator (θ = π/2) F ν has its minimum, while along the poles (θ = 0) it reaches its maximum. Numerical results of the neutrino emission from merger remnants point to Dessart et al 2009, Perego et al 2014 and are well described by α ≈ 2. The original simulation does not model the neutrino emission.…”

“…In the previous equation, t is the time with respect to the beginning of the simulation. We consider a typical cooling time scale ∆t cool ∼ 500 ms, comparable with the disk life time and with the diffusion time scale from the central MNS (e.g., Dessart et al 2009, Perego et al 2014. We note that its precise value is of small importance, since ∆t cool is much larger than t peak and the tracer expansion time scale.…”

Detectable radio flares following gravitational waves from mergers of binary neutron stars

“…Disk formation leads to several additional outflow sources. First, neutrino heating drives a wind from the disk surface (e.g., Levinson 2006;Metzger et al 2008;Dessart et al 2009). The energy in this wind is substantial with predictions ranging between 10 49 erg and 10 51 erg for 0.01-0.1 M disk.…”

The dynamic ejecta of compact object mergers and eccentric collisions

“…This type of ejecta is the main focus here. In addition, the merger remnant emits neutrinos at rates of a few times 10 53 erg s −1 [11][12][13], and this neutrino emission has been shown to drive strong baryonic winds [14]. In fact, for as long as the central, hypermassive NS has not collapsed into a BH, this baryonic wind may actually represent a serious danger for the emergence of the ultra-relativistic outflow that is required to produce a gamma-ray burst (GRB).…”