"Relativistic Jets in Gamma-Ray Burst Sources and Active Galactic Nuclei"

There is a growing recognition that magnetic fields play a dominant role in driving the collimated relativistic outflows that give rise to gamma-ray bursts (GRBs) and their afterglows. Using exact semianalytic solutions of the ``hot'' special-relativistic ideal-MHD equations, I demonstrate that a strongly magnetized accretion disk (with a dominant polodial or azimutal magnetic field) around a solar-mass black hole can efficiently accelerate a proton-electron outflow to the Lorentz factors and kinetic energies inferred in GRB sources. I discuss the role of thermal pressure (due to radiation and electron-positron pairs) in the initial acceleration of the flow and comment on possible nonideal-MHD effects. I also show that magnetic driving can significantly alleviate the ``baryon contamination'' problem in GRBs if the jet is initially neutron-rich, as expected in GRB source models. I then apply the insight gained from modeling GRB outflows to the interpretation of relativistic jets in active gala ctic nuclei. In particular, I argue that the observational indications that these jets are accelerated over distances that far exceed the scale of the central black hole imply that the acceleration is predominantly magnetic. I present preliminary results of a comprehensive model of superluminal jets that could be used to test this interpretation.