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Astron. Astrophys. 319, 122-153 (1997)


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Coalescing neutron stars - a step towards physical models

II. Neutrino emission, neutron tori, and gamma-ray bursts

M. Ruffert * 1, H.-Th. Janka ** 1, K. Takahashi *** 1, 2 and G. Schäfer **** 3

1 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, Postfach 1523, D-85740 Garching, Germany
2 Physik-Department E12, Technische Universität München, James-Frank-Str., D-85748 Garching, Germany
3 Max-Planck-Arbeitsgruppe "Gravitationstheorie", Friedrich-Schiller-Universität, Max-Wien-Platz 1, D-07743 Jena, Germany

Received 20 May 1996 / Accepted 2 July 1996

Abstract

Three-dimensional hydrodynamical, Newtonian calculations of the coalescence of equal-mass binary neutron stars are performed with the "Piecewise Parabolic Method". The properties of neutron star matter are described by the equation of state of Lattimer & Swesty (1991) which allows us to include the emission of neutrinos and to evaluate our models for the [FORMULA] -annihilation in the vicinity of the merging stars. When the stars have merged into one rapidly spinning massive body, a hot toroidal cloud of gas with a mass of about 0.1- [FORMULA] forms around the wobbling and pulsating central [FORMULA] object. At that time the total neutrino luminosity climbs to a maximum value of 1- [FORMULA]  erg/s of which 90-95% originate from the toroidal gas cloud surrounding the very dense core. The mean energies of [FORMULA], [FORMULA], and heavy-lepton neutrinos [FORMULA] are around 12 MeV, 20 MeV, and 27 MeV, respectively. The characteristics of the neutrino emission are very similar to the emission from type-II supernovae, except for the [FORMULA] luminosity from the merged neutron stars which is a factor 3-6 higher than the luminosities of the other neutrino species.

When the neutrino luminosities are highest, [FORMULA] -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- [FORMULA]  erg/s. Since the [FORMULA] 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 [FORMULA] -ray bursts at cosmological distances with an energy of the order of [FORMULA]  erg/steradian. Analytical estimates show that the additional energy provided by the annihilation of [FORMULA] pairs emitted from a possible accretion torus of [FORMULA] 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. A few [FORMULA] of very neutron-rich, low-entropy matter may be dynamically ejected shortly after the neutron stars have merged, and another [FORMULA] up to a few [FORMULA] of strongly neutronized, high-entropy material could be carried away from the accretion torus in a neutrino-driven wind. The contamination with this baryonic material is a severe threat to a relativistic fireball. Aspects of a possible r-processing in these ejecta are discussed.

Key words: gamma rays: bursts – elementary particles: neutrinos – nuclear reactions, nucleosynthesis, abundances – stars: neutron – binaries: close – hydrodynamics

* mruffert@mpa-garching.mpg.de
** thj@mpa-garching.mpg.de
*** kjt@mpa-garching.mpg.de
**** gos@gravi.physik.uni-jena.de

Send offprint requests to: M. Ruffert

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998

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