Magnetic Fields, Accretion, and the Central Engine of Gamma–Ray Bursts

I briefly review magnetic effects in accretion physics, and then go on to discuss a possible central engine for gamma–ray bursts. A rotating black hole immersed in a non–axisymmetric magnetic field experiences a torque trying to align spin and field. I suggest that gamma–ray burst hosts may provide conditions where this effect allows rapid extraction of a significant fraction of the hole’s rotational energy. I argue that much of the electromagnetic emission is in two narrow beams parallel and antiparallel to the asymptotic field direction. This picture suggests that only a mass ∼ 10M⊙ is expelled in a relativistic outflow, as required by the fireball picture.