  The collapse of young massive stars or the coalescence of a black
hole-neutron star binary is expected to give rise to a black hole-torus system.
When the torus is strongly magnetized, the black hole produces
electron-positron outflow along open magnetic field-lines. Through curvature
radiation in gaps, this outflow rapidly develops into a $e^\pm\gamma$-wind,
which is ultra-relativistic and of low comoving density, proposed here as a
possible input to GRB fireball models.
