By means of three-dimensional hydrodynamic simulations with a Eulerian PPM code we investigate the formation and the properties of the accretion torus around the stellar mass black hole which originates from the merging of two neutron stars .
The simulations are performed with four nested cartesian grids which allow for both a good resolution near the central black hole and a large computational volume .
They include the use of a physical equation of state as well as the neutrino emission from the hot matter of the torus .
The gravity of the black hole is described with a Newtonian and alternatively with a Paczyński-Wiita potential .
In a post-processing step , we evaluate our models for the energy deposition by \nu \bar { \nu } annihilation around the accretion torus .

We find that the torus has a mass between several 10 ^ { -2 } M _ { \odot } and a few 10 ^ { -1 } M _ { \odot } with maximum densities around 10 ^ { 12 } { g cm } ^ { -3 } and maximum temperatures of about 10 MeV ( entropies around 5 k _ { B } per nucleon ) .
Correspondingly , the neutrino emission is huge with a total luminosity near 10 ^ { 53 } { erg s } ^ { -1 } .
Neutrino-antineutrino annihilation deposits energy in the vicinity of the torus at a rate of ( 3– 5 ) \times 10 ^ { 50 } { erg s } ^ { -1 } .
It is most efficient near the rotation axis where 10 to 30 % of this energy or up to a total of 10 ^ { 49 } { erg } are dumped within an estimated emission period of 0.02–0.1 s in a region with a low integral baryonic mass of about 10 ^ { -5 } M _ { \odot } .
This baryon pollution is still dangerously high , and the estimated maximum relativistic Lorentz factors \Gamma - 1 are around unity .
The conversion of neutrino energy into a pair plasma , however , is sufficiently powerful to blow out the baryons along the axis so that a clean funnel should be produced within only milliseconds .
Our models show that \nu \bar { \nu } annihilation can yield the energy to account for weak , short gamma-ray bursts , if moderate beaming is involved .
In fact , the barrier of the dense baryonic gas of the torus suggests that the low-density e ^ { \pm } \gamma plasma is beamed as axial jets into a fraction f _ { \Omega } = 2 \delta \Omega / ( 4 \pi ) between 1 / 100 and 1 / 10 of the sky , corresponding to opening half-angles of roughly ten to several tens of degrees .
Thus \gamma -burst energies of E _ { \gamma } \approx E _ { \nu \bar { \nu } } / f _ { \Omega } \mathrel { \vbox { \offinterlineskip% \hbox { $ < $ } \hbox { $ \sim$ } } } 10 ^ { 50 } – 10 ^ { 51 } erg seem within the reach of our models ( if the source is interpreted as radiating isotropically ) , corresponding to luminosities around 10 ^ { 51 } { erg s } ^ { -1 } for typical burst durations of 0.1–1 s. Gravitational capture of radiation by the black hole , redshift and ray bending do not reduce the jet energy significantly , because most of the neutrino emission comes from parts of the torus at distances of several Schwarzschild radii from the black hole .
Effects associated with the Kerr character of the rapidly rotating black hole , however , could increase the \gamma -burst energy considerably , and effects due to magnetic fields might even be required to get the energies for long complex gamma-ray bursts .