  A hyperaccretion disk formed around a stellar mass black hole is a plausible
model for the central engine that powers gamma-ray bursts (GRBs). If the
central black hole rotates and a poloidal magnetic field threads its horizon, a
powerful relativistic jet may be driven by a process resembling the
Blandford-Znajek mechanism. We estimate the luminosity of such a jet assuming
that the poloidal magnetic field strength is comparable to the inner accretion
disk pressure. We show that the jet efficiency attains its maximal value when
the accretion flow is cooled via optically-thin neutrino emission. The jet
luminosity is much larger than the energy deposition through
neutrino-antineutrino annihilation provided that the black hole is spinning
rapidly enough. When the accretion rate onto a rapidly spinning black hole is
large enough (> 0.003-0.01M_sun/sec), the predicted jet luminosity is
sufficient to drive a GRB.
