Astron. Astrophys. 324, 461-470 (1997)

2. Magnetized accretion disks in active galactic nuclei

Active galactic nuclei (AGN) show extremely energetic outflows extending even to scales beyond the outer edge of the galaxy in the form of strongly collimated radio jets (Bridle & Perley 1984; Perley 1989). There is substantial evidence that magnetic forces are involved in the jet driving mechanism (Blandford & Payne 1982; Blandford 1989; Camenzind 1990a, b) and that the magnetic fields will also provide the collimation of the flow, since huge currents are involved in the jet flows (Benford 1978; Heyvarts & Norman 1989; Lesch et al., 1989).

A Keplerian accretion disk has been found beyond any doubt in the mildly active galaxy NGC 4258 (Miyoshi et al. 1995). Additionally, at least in some cases the AGN show some evidence for rotating accretion disks. These objects exhibit the classical double-peaked, broad emission lines which are considered to be characteristic for a Keplerian rotating disk. Standard thin disks do, however, not release sufficient gravitational energy locally at the radii, where the Keplerian velocity corresponds to the observed width of the emission lines (typically at Schwarzschild radii). Some extra source of energy is required (Collin-Souffrin 1987), which could be found in the interaction of an accretion disk with the corona of the AGN.

Therefore, we consider as a possible scenario a rotating black hole surrounded by a magnetized accretion disk (Camenzind 1990a, b). The origin of the magnetic fields in accretion disks must be due to the existence of seed fields in the disks which extend to larger scales of a few parsecs to a few hundreds of parsecs in galaxies (Begelman et al. 1989). These larger disks and rings represent reservoirs for gas and magnetic flux which are accreted onto the central black hole.

As the plasma of the disk is accreted in the gravitational potential of the central object, magnetic field lines are convected inwards, amplified and finally deposited at the horizon of the black hole. A dynamo in the disk may be responsible for the maintenance and amplification of the magnetic field in the disk (for the details of dynamo action in accretion disks, see Khanna and Camenzind (1994). The interplay of differential rotation and convective turbulence (ascending (descending) turbulent cells) amplifies the magnetic field whereas processes like magnetic buoyancy limit further amplification (Stella & Rosner 1984). An inevitable consequence of convective turbulence and magnetic buoyancy is the formation of a hot, magnetically active corona due to the transfer of magnetic energy to the coronal plasma (see Fig. 1).

Fig. 1. Hydromagnetic phenomena in thin accretion disks and the buildup of a corona. Whenever the magnetic field in the disk (generated by a dynamo) is in equipartition with the amplifying forces (differential shear and convective motions), magnetic flux tubes, convective turbulent cells, etc. will ascend and feed magnetic energy into the corona. If the driving forces cannot be counteracted by normal magnetic stresses in the disk, the field lines are bent in the direction of the force and currents parallel to the magnetic field propagate into the corona. To close the current circuit it is necessary to drive perpendicular currents somewhere in the corona. Moreover, in order for a current to flow, a resistor (load) within the circuit is necessary (Ohm's law). This leads to localized dissipative regions, where magnetic energy is transferred into particle energy via field-aligned electric fields.

The significance of coronal dissipation is clearly indicated by X-rays and -rays observations of quasars (Maraschi et al. 1989; Dermer and Schlickeiser 1993) which show that the emission cannot originate in a thin disk since the disk temperatures do not exceed (Straumann 1986), but must be emitted in the corona and jet, respectively. The UV disk photons are Compton scattered by the hot or relativistic coronal electrons and positrons. Zdiarski et al. (1993) argue that the observations of the Seyfert galaxy NGC 4151 in hard X-rays and in -rays are well explained by a nonthermal model with acceleration of relativistic electrons at an effiency of . The particles loose energy via inverse Compton scattering of UV photons and are continuously reaccelerated to Lorentz factors of a few hundred. We note again that such an acceleration is not possible via Fermi-processes, because the necessary energies of the particles are below . We emphasize that the observed flux variations in the optical and UV ranges are too fast to be caused by the variations of the accretion rate only.

It was proposed (Ulrich 1991) that instabilities at the inner part of the disk are a possible origin for these fast variations. Evidences for localized dissipation regions are also given by global models for the origin of relativistic jets in AGN (Camenzind 1990a). Such models contain a rotating, supermassive black hole, surrounded by a magnetized accretion disk and a corona. The relativistic jet velocities are caused by magnetic acceleration and the collimation of the outflows is produced by currents flowing along the jet axis. If the electric current is resolved into a field-aligned current and a current perpendicular to the magnetic field , one can relate to by on the grounds of current conservation. In the considered scenario the plasma in the corona is coupled electrodynamically with the plasma in the accretion disk by field-aligned electric currents. therefore represents the means by which the magnetic energy is transported from the generator (the disk with ) to its "storage" site. The generation of perpendicular currents in the accretion disk is the result from mechanical forces of the differential shear acting on the disk magnetic field. Whenever the magnetic field in the disk (generated by a dynamo) is in equipartition with the amplifying forces (differential shear and turbulent motions) magnetic flux tubes, convective turbulent cells, etc. will ascend, which by driving field-aligned currents feed magnetic energy into the corona. In other words, if the driving forces cannot be balanced by normal magnetic stresses in the disk the field lines are bent in the direction of the force and currents parallel to the magnetic field propagate into the corona (see Fig. 1). The closure of the current circuit will be provided alternatively by flux tubes bending back to the disk surface or by perpendicular currents in the corona.

Summarizing, if a differentially rotating accretion disk accretes magnetized gas, the formation of an electrodynamically coupled corona is unavoidable. The transfer of shear stresses is effected via field-aligned currents. A concentration of electric energy in thin sheets increases the current density. The formation of field-aligned potential drops depends sensitively on the actual value of the current density.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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