Annu. Rev. Astron. Astrophys. 1988. 36: 539-598 Copyright © 1998 by Annual Reviews. All rights reserved

7. OPEN PROBLEMS

Much progress has been made in modeling extragalactic jets in the last few years owing to (a) the quantitative and qualitative enrichment of the statistical sample of detailed multifrequency observations and (b) the development of reliable numerical codes to simulate the microphysics of supersonic and relativistic outflows. The basic results definitively acquired at present are as follows:

1. Jet acceleration and collimation take place in the inner regions ofAGNs through processes in which magnetic fields anchored in accretion disks are the fundamental elements. The disk-jet connection is also operating in other astrophysical conditions, such as binary active stars and star formation regions.
2. Jet propagation survives dynamical and kinetic instabilities due to the interaction with the ambient medium owing to nonlinear effects that create turbulent boundary layers and overpressured cocoons around them.
3. Jet morphologies can be interpreted in terms of the above instabilities connecting shock formation, suprathermal particle acceleration, and synchrotron emission with bright knots, hot spots, bow shocks, and cocoons. The distinction between FR I and II radio galaxies is related to the energy dissipation inside jets, which is parameterized in terms of the flow Mach number and the density contrast with the ambient medium (or cocoon).
4. Doppler beaming is involved in the strong variability of quasars and blazars and may also explain the one-sidedness of strong jets and the global energetics of -ray AGNs. This result is, however, still preliminary.

On the other hand, many questions remain open, and more investigations are needed to settle the physics governing fundamental phenomena:

1. Does the bulk flow contain an ion/electron plasma or anelectron/positron pair plasma? Originally, the pair plasma was seen as a factor in reducing global energetic demands. Now the question is how efficiently relativistic bulk flows can be produced.
2. How do accretion disks launch collimated flows? Magnetized coronae heated by reconnection of loops buoyant from the accretion disk are perhaps the best candidates, but other processes have not been fully investigated yet, in particular those involving electromagnetic forces and currents. Nor has much progress been made on the (formidably difficult indeed) study of the electrodynamics of black hole magnetospheres.
3. Is a fluid approximation a reliable way to represent the microphysics of jets? Most likely not: Kinetic effects define the transport coefficients and the development of perturbations and nonlinear structures inside the flow. Therefore, the boundary layer at the interface between jets and ambient medium is certainly governed by these coefficients, and so far we have only preliminary indications about their effects on mixing, entrainment, turbulence, etc. A fully kinetic treatment is at present prohibitive; the next possible step may be a two-fluid model implemented in numerical codes that have been adapted to the problem.
4. How important are currents? Given the high conductivity of astrophysical plasmas, the general trend is to neglect charge separation and current effects on large scales. Low return currents can be dispersed over large cocoons with low current densities without perturbing the ambient medium. However, current densities may be very high inside jets and may again influence the transport coefficients and dissipation processes. The above-mentioned two-fluid approach might answer this question.
5. We have very little information about the spatial and spectral distributions of the suprathermal component of relativistic electrons and ions. From the theoretical side, not much effort has been made so far to couple the suprathermal and thermal components and to couple both of them to the emitted radiation.

If a conclusion can be drawn at this stage of modeling astrophysical jets, it might be said that, while 10 years ago most of what had been observed was interpreted at a phenomenological level, today we have a more quantitative understanding. The general picture is rather firm, but most of the microphysics involved are still unsatisfactorily implemented.

Acknowledgments

This work was supported by grants of the Italian Ministero dell'Università (MURST) at the Osservatorio Astronomico di Torino and by hospitality at the University of California at Irvine and at the Department of Astronomy & Astrophysics of the University of Chicago. The author wishes to thank many colleagues who have shared their interest in the challenging questions of astrophysical jets, in particular, Gregory Benford, Gianluigi Bodo, Silvano Massaglia, Robert Rosner, Paola Rossi, Edoardo Trussoni, and Kanaris, Tsinganos.