Jet Formation and Collimation

Abstract

We briefly review our current understanding for the formation, acceleration and collimation of winds to jets associated with compact astrophysical objects such as AGN and µQuasars.All such outflows may be considered to a first approximation as ideal MHD plasmas escaping from a rotating and magnetized accretion disk with a magnetosphere around a central black hole. A crucial ingredient for a correct modelling of the steady state problem is to place the appropriate boundary conditions, by taking into account how information can propagate through the outflow and ensuring, e.g., that shocks produced via the interaction of the flow with the external medium do not affect the overall structure. As an example underlining the role of setting the correct boundary conditions, we make the analogy of the critical surfaces in the steady and axisymmetric MHD problem with the event horizon and ergosphere of a rotating black hole in relativity.We discuss the acceleration of the outflow, by gas, radiation, or wave pressure gradients and also by magnetic mechanisms, showing the important role played by the disk corona in the vicinity of the black hole. Pressure and magnetic confinement both may also play a role in confining the outflow, although magnetic hoop stress confinement is likely to be a rather dominant process in tightly collimated outflows. The possible asymptotical morphology that jets achieve and the instabilities which are likely to explain the observed structures but do not prevent jets to possess toroidal magnetic fields are also reviewed.Finally, it is proposed that in a space where the two main variables are the energy of the magnetic rotator and the angle between the line of sight and the ejection axis, some observed characteristics of AGN jets can be understood. A criterion for the transition of the morphologies of the outflows from highly collimated jets to uncollimated winds is given. It is based on the analysis of a particular class of exact solutions and may somehow generalize other earlier suggestions, such as the spinning of the black hole, the fueling of the central object, or the effects of the environment.Thus, while the horizontal AGN classification from Type 0 to Types 1 and 2 may well be an orientation effect - i.e. a dependence on the viewing angle between the source axis and the observer as in the standard model - the vertical AGN classification with uncollimated outflows (radio-quiet sources) and collimated outflows (radio-loud sources) depends both on the efficiency of the magnetic rotator and the environment in which the outflows propagate.