Coalescing neutron stars -- a step towards physical models. II. Neutrino emission, neutron tori, and gamma-ray bursts

TL;DR

This paper presents 3D hydrodynamical simulations of neutron star mergers, analyzing neutrino emissions, torus formation, and implications for gamma-ray bursts, concluding that neutrino annihilation alone is insufficient to power typical cosmological GRBs.

Contribution

It provides detailed modeling of neutrino emission and energy deposition in neutron star mergers, highlighting limitations for gamma-ray burst production.

Findings

Neutrino luminosity peaks at 1-1.5×10^{53} erg/s.

Neutrino annihilation deposits only 0.2-0.3% of neutrino energy.

Energy from neutrino annihilation is insufficient for typical GRBs.

Abstract

Three-dimensional hydrodynamical, Newtonian calculations of the coalescence of equal-mass binary neutron stars are performed, including a physical high-density equation of state and a treatment of the neutrino emission of the heated matter. The total neutrino luminosity climbs to a maximum value of 1--$1.5\cdot 10^{53}$~erg/s of which 90--95\% originate from the toroidal gas cloud surrounding the very dense core formed after the merging. When the neutrino luminosities are highest, $\nu\bar\nu$-annihilation deposits about 0.2--0.3\% of the emitted neutrino energy in the immediate neighborhood of the merger, and the maximum integral energy deposition rate is 3--$4\cdot 10^{50}$~erg/s. Since the $3\,M_{\odot}$ core of the merged object will most likely collapse into a black hole within milliseconds, the energy that can be pumped into a pair-photon fireball is insufficient by a factor of about 1000 to explain $\gamma$-ray bursts at cosmological distances with an energy of the order of $10^{51}/(4\pi)$~erg/steradian. Analytical estimates show that the additional energy provided by the annihilation of $\nu\bar\nu$ pairs emitted from a possible accretion torus of $\sim 0.1\,M_{\odot}$ around the central black hole is still more than a factor of 10 too small, unless focussing of the fireball into a jet-like expansion plays an important role. About $10^{-4}$--$10^{-3}$~$M_\odot$ of material lost during the neutron star merging and swept out from the system in a neutrino-driven wind might be a site for nucleosythesis. Aspects of a possible r-processing in these ejecta are discussed.