Annu. Rev. Astron. Astrophys. 1999. 37: 409-443
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6. JET FORMATION

The processes by which the jets are accelerated and collimated are still not clearly understood, but it is believed that several of the concepts proposed for extragalactic jets can be extended to galactic jets.

Blandford & Znajek (1977) take advantage of the fact that, in principle, it is possible to extract energy and angular momentum from a rotating black hole (Penrose 1969), to produce electric and magnetic fields and possibly fast outflowing jets. A magnetized accretion disk around the Kerr black hole brakes it electromagnetically. However, Ghosh & Abramowicz (1997), Livio et al (1997) have called into question that the Blandford-Znajek process can provide the primary power in the jets.

A seminal idea that has been followed by many researchers in the field is that of the magnetohydrodynamical model of Blandford & Payne (1982). These authors proposed that the angular momentum of a magnetized accretion disk around the collapsed object is the responsible for the acceleration of the plasma. The magnetic field lines are taken to be frozen into the disk and the plasma is assumed to follow them like a "bead on a wire", at least close to the disk. If the field line forms an angle with the plane of the disk smaller than 60°, the displacements of the plasma from its equilibrium position become unstable. This happens because along these field lines the component of the centrifugal force will be larger than the component of the gravitational force and the plasma will be accelerated outwards. Then, in its origin, the outflow motion has an important "equatorial" component, while on larger scales the jets are observed to have a motion that is dominantly "poloidal". In other words, after the acceleration a collimating mechanism is required to change the wide-angle centrifugal outflow into a collimated jet.

This collimation is proposed to be achieved as follows. Inside an inner region, the magnetic field energy density is larger than the kinetic energy density of the flow but at some distance from the disk (the Alfvén surface), this situation reverses and the flow stops corotating with the disk. This causes a loop of toroidal (azimuthal) field to be added to the flow for each rotation of the footpoint of the field line. The tension of this wound-up toroidal field that is formed external to the Alfvén surface produces a force directed toward the axis (the "hoop stress") that eventually collimates the flow into a jet. Most models for the production of jets in the astrophysical context use elements of MHD acceleration and collimation.

Recently, several groups (Spruit et al 1997, Lucek & Bell 1997, Begelman 1998) have pointed out that the toroidal field traditionally held responsible for collimating jets in the MHD mechanism is unstable and cannot collimate the jets effectively. It has been proposed alternatively that the collimating agent is the poloidal component of the magnetic field.

Koide et al (1998) have performed for the first time full general relativistic MHD numerical simulations of the formation of jets near a black hole. Their results suggest that the ejected jet has a two-layer structure with an inner, fast gas-pressure driven component and an outer, slow magnetically driven component. The presence of the inner, fast gas-pressure driven component is a result of the strong pressure increase produced by shocks in the disk through fast advection flows inside the last stable orbit around a black hole. This feature is not seen in non-relativistic calculations.

Within the uncertainties of the small sample, the velocity of the jets seems to show a bimodal distribution, with some sources having vjet appeq 0.3c and others having vjet geq 0.9c. Two explanations have been offered in the literature. On one hand, Kudoh & Shibata (1995) suggest that the terminal velocity of the jet is of order of the Keplerian velocity at the footpoint of the jets, that is, that the fastest jets probably come from the deepest gravitational wells (Livio 1997). However, recent observations suggest that Sco X-1 which is a neutron star binary has vjet ~ 0.5c (Fomalont 1999), departing from the bimodal distribution. On the other hand, Meier et al (1997) propose that the velocity of the jets is regulated by a magnetic "switch", with highly relativistic velocities achieved only above a critical value of the ratio of the Alfvén velocity to the escape velocity. The determination of the mass of the collapsed object in a larger number of jet sources would discriminate between these two models.

While it seems that a steady state MHD model can account for the formation of continuous relativistic jets, the events discussed by Mirabel et al (1998), Belloni et al (1998), and Fender & Pooley (1998) that seem to involve a connection between the disappearance of the inner accretion disk and the sudden ejection of condensations may require a different mechanism. Clearly, the time seems to be ripe for new theoretical advances on the models of formation of relativistic jets that take into account the observational features found in stellar jets.

Another characteristic that the jet models must account for is the production of relativistic particles that will produce the synchrotron emission that is observed in several sources. As in other astrophysical contexts, it is believed that the acceleration of electrons to relativistic speeds takes place in shocks (Blandford & Ostriker 1978). On the other hand, most of the X-ray binaries are "radio-quiet", implying that relativistic electrons and/or magnetic fields are not always present in sufficient amounts.

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