2005A&A...436..273A

We present the first general relativistic hydrodynamic models of the launch and evolution of relativistic jets and winds, driven by thermal energy deposition, possibly due to neutrino-antineutrino annihilation, in the close vicinity of black hole-accretion torus systems. The latter are considered to be the remnants of compact object mergers. Our two-dimensional simulations establish the link between models of such mergers and future observations of short gamma-ray bursts by the SWIFT satellite. They show that ultrarelativistic outflow with maximum terminal Lorentz factors around 1000 develops for polar energy deposition rates above some 10⁴⁸erg/s per steradian, provided the merger environment has a sufficiently low baryon density. By the interaction with the dense accretion torus the ultrarelativistic outflow with Lorentz factors Γ above 100 is collimated into a sharp-edged cone that is embedded laterally by a wind with steeply declining Lorentz factor. The typical semi-opening angles of the Γ>100 cone are 5°-10°, corresponding to about 0.4-1.5% of the hemisphere and apparent isotropized energies (kinetic plus internal) up to ≃10⁵¹erg although at most 10-30% of the deposited energy is transferred to the outflow with Γ>100. The viability of post-merger black hole-torus systems as engines of short, hard gamma-ray bursts is therefore confirmed. The annihilation of neutrino-antineutrino pairs radiated from the hot accretion torus appears as a suitable energy source for powerful axial outflow even if only ≃10⁴⁹erg are deposited within a cone of 45° half-opening angle around the system axis. Although the torus lifetimes are expected to be only between some 0.01s and several 0.1s, our models can explain the durations of all observed short gamma-ray bursts, because different propagation velocities of the front and rear ends will lead to a radial stretching of the ultrarelativistic fireball before transparency is reached. The ultrarelativistic flow reveals a highly non-uniform structure caused by the action of Kelvin-Helmholtz instabilities that originate at the fireball-torus interface. Large radial variations of the baryon density (up to several orders of magnitude) are uncorrelated with moderate variations of the Lorentz factor (factors of a few) and fluctuations of the gently declining radiation-dominated pressure. In the angular direction the Lorentz factor reveals a nearly flat plateau-like maximum with values of several hundreds, that can be located up to 7° off the symmetry axis, and a steep decrease to less than 10 for polar angles larger than 15°-20°. Lateral expansion of the ultrarelativistic core of the flow is prevented by a subsonic velocity component of about 0.05c towards the symmetry axis, whereas the moderately relativistic wings show a subsonic sideways inflation with less than 0.07c (measured in the frame comoving with the radial flow).