Runaway Instability of Black Hole--Neutron Torus Systems

We have obtained fully general relativistic equilibrium configurations of black hole-neutron torus systems and have analyzed their stability. Although several authors have discussed whether such systems can be sources of gamma-ray bursts, their arguments have been based on the qualitative nature of black hole-neutron torus systems because such configurations have not yet been solved. Moreover, since black hole-neutron torus systems are supposed to form after coalescence of binary neutron stars or after collapse of a massive star, a newly formed torus may be in a rather hot state. Therefore we have developed a numerical code that can handle configurations of cold and hot (0.1, 1, and 10 MeV) neutron tori. Recently, polytropic tori around black holes have been shown to suffer a global runaway instability. As for neutron tori, the self-gravitating tori closely orbiting around the black holes with the mass range of 1-3 Msun are destroyed in a timescale ΔtTτ ≪ 1 s as a result of this instability. This is a much shorter timescale than the evolutionary timescale that is relevant for the current model of gamma-ray bursts based on the merger of binary neutron stars. This result probably excludes the possibility of neutron tori models for gamma-ray bursts as sources at cosmological distances.