  We investigate the possibility that gamma-ray bursts are powered by a central
engine consisting of a black hole with an external magnetic field supported by
a surrounding disk or torus. The rotational energy of the black hole can be
extracted electromagnetically as a Poynting flux, a mechanism proposed by
Blandford and Znajek(1977). Recently observed magnetars indicate that some
compact objects have very high magnetic fields, up to $10^{15}$ G, which is
required to extract the energy within the duration of a GRB, i.e., in 1000 s or
less. We demonstrate also that the Poynting flux need not be substantially
dominated by the disk.
