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 νν-annihilation near the disc is evaluated in a postprocessing step. Our simulations show that the neutrino emission and energy deposition by νν-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, νν-annihilation can be a viable energy source for gamma-ray bursts.

By means of three-dimensional hydrodynamic simulations with an Eulerian PPM code we investigate the time-dependent evolution and properties of accretion tori around nonrotating and rotating stellar-mass black holes, using a pseudo-Newtonian (Paczyński & Wiita or Artemova-Björnsson-Novikov) potential to approximate the effects of general relativity. The simulations are performed with three nested Cartesian grids to ensure sufficient resolution near the central black hole on the one hand and a large computational volume on the other. The black hole and torus are considered as the remnant of a binary neutron star or neutron-star black-hole merger. Referring to results from previous hydrodynamical simulations of such events, we assume the initial configurations to consist of a black hole with a mass of about 4 M girded by a toroidal accretion disk with a mass in the range from about 0.01 M to 0.2 M . We simulate the torus evolution without and with physical shear viscosity, employing a simple α-model for the gas viscosity. As in our previous work on merging neutron star binaries and neutron star/black hole binaries, we use the equation of state of Lattimer and Swesty. The energy loss and lepton number change due to neutrino emission from the hot torus are treated by a neutrino-trapping scheme. The energy deposition by neutrino-antineutrino annihilation around the disk is evaluated in a post-processing step. The timedependent efficiency of converting gravitational energy to neutrinos, expressed by the ratio of neutrino luminosity to accretion rate of rest-mass energy, can reach maximum values of up to about 10%. The corresponding efficiency of converting neutrino energy into a pair-photon fireball by neutrino annihilation peaks at values of several percent. Interestingly, we find that the rate of neutrinoantineutrino annihilation decays with time much less steeply than the total neutrino luminosity does with the decreasing gas mass of the torus, because the ongoing protonization of the initially neutron-rich disk matter leads to a rather stable product of neutrino and antineutrino luminosities. The neutrino luminosity and total energy release of the torus increase steeply with higher viscosity, larger torus mass, and larger black hole spin in corotation with the disk, in particular when the spin parameter is a > ∼ 0.8. The latter dependence is moderated in case of a high disk viscosity. For rotation rates as expected for post-merger black holes (a > ∼ 0.5) and reasonable values of the alpha viscosity of the torus (α ∼ 0.1), torus masses in the investigated range can release sufficient energy in neutrinos to account for the energetics of the well-localized short gamma-ray bursts recently detected by Hete and Swift, if collimation of the ultrarelativistic outflows into about 1% of the sky is invoked, as predicted by recent hydrodynamic jet simulations.

Abstract. Gravitation has interesting consequences for electromagnetic wave propagation in vacuum. The propagation of plane waves with phase velocity directed opposite to the time-averaged Poynting vector is investigated for a generally curved spacetime. Conditions for such negative-phase-velocity (NPV) propagation are established in terms of the spacetime metric components for general and special cases. Implications of the negative energy density of NPV propagation are discussed.

Consistently with the Einstein equivalence principle and using an electromagnetic formulation first suggested by Tamm, we show that a local observer cannot observe negative-phase-velocity (NPV) propagation of electromagnetic waves in vacuum, whereas a global observer can appreciate that phenomenon. Using the specific example of the Kerr metric, we also demonstrate the possibility of NPV propagation within the ergosphere of a rotating black hole.

The propagation of electromagnetic plane waves with negative phase velocity (NPV) is considered in Schwarzschild-(anti-)de Sitter spacetime. It is demonstrated that NPV propagation occurs in Schwarzschild-de Sitter spacetime at lower values of the cosmological constant than is the case for de Sitter spacetime. Furthermore, we report that neither is NPV propagation observed in Schwarzschild-anti-de Sitter spacetime, nor is it possible outside the event horizon of a Schwarzschild blackhole.