Poynting Jets from Black Holes and Cosmological Gamma-Ray Bursts

We discuss the properties of magnetically dominated jetlike outflows from stellar mass black holes surrounded by debris tori resulting from neutron star disruption. These jets may have narrow cores (along the rotation axis) that are almost free of baryons and attain very high bulk Lorentz factors ≳106. The jets give rise to a characteristic MeV to TeV emission as well as to relativistic shocks producing the usual MeV bursts. Because the outflow is highly directional, the properties of the observed gamma rays would depend on the viewing angle relative to the rotation axis. Even for the most intense bursts, which under the assumption of isotropic emission and substantial redshifts would be inferred to emit 1052-1053 ergs, the efficiencies required are only 10-2 to 10-4.