Active Galactic Nuclei

Although a wide variety of astrophysical objects produce powerful jets, we still lack a comprehensive theory of their formation. In our Galaxy, young stellar objects, massive X-ray binaries, black hole X-ray transients, symbiotic stars, supersoft X-ray sources and even some planetary nebulae are all accreting systems that produce jets.

First, there is strong observational evidence that almost all jet-producing systems contain an 'accretion disc' around a compact object. This disc is both a source of energy (in the compact object's gravitational potential) and provides the required axial symmetry. Second, jet velocity is always of the order of the escape velocity from the central object, indicating that jets originate from the centre of the accretion disc. The Galactic black hole transient source GRS 1915 + 105 (a microquasar) provides even more evidence for the connection between the jet and the central part of the disc, and we can begin to see a picture in which the inner disc is episodically accreted while ejecting a relativistic plasma, which subsequently produces infrared and radio flares by synchrotron emission.

The most promising universal mechanism for jet acceleration and collimation relies on an accretion disc threaded by a poloidal, large-scale magnetic field. The basic idea is that some magnetic flux is in open field lines, which form a certain angle with the disc's surface. Ionized material is forced to follow field lines; as these lines are anchored in the disc and rotate with it, material is centrifugally accelerated like a bead on a wire. The jet may be collimated by hoop stresses resulting from the magnetic field's toroidal component; alternatively, if the poloidal magnetic flux is largest at the disc's edge, and the disc's radius is large relative to that of its central object, the jet will be collimated by the magnetic field lines.

(After Livio 2002.)