A MAGNETIC SWITCH THAT DETERMINES THE SPEED OF ASTROPHYSICAL JETS

The mechanism by which astrophysical jets form is an important factor in understanding the nature and evolution of phenomena such as active galactic nuclei and quasars, Galactic superluminal X-ray sources and y oung stellar objects. Of the many schemes proposed for jet production, only the magnetized accretion disk model of Blandford and Payne(1) se ems to be applicable to all of these systems, and also offers the pote ntial for generating the highly relativistic flows observed in some qu asars(2). But the source of variation in jet morphology observed for d ifferent sources remains unclear. Here we report time-dependent numeri cal simulations of jet formation which show that the character and spe ed of the jets produced depend dramatically on whether magnetic forces dominate over gravity in the accretion disk corona. This 'magnetic sw itch' is not predicted by steady-state, self-similar disk models, or b y relativistic wind theory (which generally ignores the gravitational field). The effect provides a natural explanation for the existence of two known classes of extragalactic radio source and for the variation of their properties with radio luminosity. It also provides insight i nto protostellar and galactic microquasar systems.