Non-stationary hyperaccretion of stellar-mass black holes in three dimensions: torus evolution and neutrino emission

We present three-dimensional hydrodynamic simulations of the evolution of self-gravitating, thick accretion discs around hyperaccreting stellar-mass black holes. The black hole–torus systems are considered to be remnants of compact object mergers, in which case the disc is not fed by an external mass reservoir and the accretion is non-stationary. Our models take into account viscous dissipation, described by an α-law, a detailed equation of state for the disc gas, and an approximate treatment of general relativistic effects on the disc structure by using a pseudo-Newtonian potential for the black hole including its possible rotation and spin-up during accretion. Magnetic fields are ignored. The neutrino emission of the hot disc is treated by a neutrino-trapping scheme, and the -annihilation near the disc is evaluated in a post-processing step. Our simulations show that the neutrino emission and energy deposition by -annihilation increase sensitively with the disc mass, with the black hole spin in case of a disc in corotation, and in particular with the α-viscosity. We find that for sufficiently large α-viscosity, -annihilation can be a viable energy source for gamma-ray bursts.