Theory of magnetically powered jets

TL;DR

This paper reviews the magnetic theory of jet production by accreting objects, discussing mechanisms, simulation interpretations, and the role of magnetic fields, with emphasis on unresolved issues and three-dimensional effects.

Contribution

It provides a comprehensive review of magnetic jet theory, highlighting key problems, clarifying terminology, and emphasizing the importance of magnetic dissipation, collimation, and 3D effects.

Findings

Magnetic energy dissipation is crucial for achieving high Lorentz factors.

Jet collimation involves complex magnetic stresses, with 'hoop stress' being a debated factor.

Strong, ordered magnetic fields may determine jet presence and X-ray binary behavior.

Abstract

The magnetic theory for the production of jets by accreting objects is reviewed with emphasis on outstanding problem areas. An effort is made to show the connections behind the occasionally diverging nomenclature in the literature, to contrast the different points of view about basic mechanisms, and to highlight concepts for interpreting the results of numerical simulations. The role of dissipation of magnetic energy in accelerating the flow is discussed, and its importance for explaining high Lorentz factors. The collimation of jets to the observed narrow angles is discussed, including a critical discussion of the role of `hoop stress'. The transition between disk and outflow is one of the least understood parts of the magnetic theory; its role in setting the mass flux in the wind, in possible modulations of the mass flux, and the uncertainties in treating it realistically are discussed. Current views on most of these problems are still strongly influenced by the restriction to 2 dimensions (axisymmetry) in previous analytical and numerical work; 3-D effects likely to be important are suggested. An interesting problem area is the nature and origin of the strong, preferably highly ordered magnetic fields known to work best for jet production. The observational evidence for such fields and their behavior in numerical simulations is discussed. I argue that the presence or absence of such fields may well be the `second parameter' governing not only the presence of jets but also the X-ray spectra and timing behavior of X-ray binaries.