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Astron. Astrophys. 319, 1025-1035 (1997)

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5. Conclusion

In this work, we presented numerical solutions of the 2D force balance equation for strongly magnetized jets originating from the inner part of an accretion disk surrounding a black hole. The calculations were performed on the background of Kerr geometry.

The model topology underlying the calculations basically follows the standard model for AGN. The jet magnetosphere originates very close to the black hole from an accretion disk. In the solutions presented here, the field rotates rigidly with a fraction of the rotational velocity of the hole. The solutions are global solutions extending from the black hole's inner light surface to an asymptotic jet at a distance of 50 horizon radii.

The asymptotic jet is collimated to a cylindrical shape in agreement with the high degree of collimation observed for extragalactic jets. With the chosen parameters for the rotation, the asymptotic jet radius is 3 light cylinder radii or 30 horizon radii. For a black hole mass of [FORMULA], this corresponds to a jet radius of [FORMULA]. We found indications for an upper limit for the asymptotic jet radius for a force-free, cylindrical jet of about 4 light cylinder radii.

The solutions satisfy the regularity condition at the light surfaces and cross the outer light surface smoothly, i.e. without unphysical kinks in the field lines. The matching across this critical surface is achieved by a proper iterative adjustment of the current distribution and the shape of the jet boundary. It therefore determines the shape of the jet in the collimation region.

The field distribution near the disk (the 'corona') is directly influenced by the disk flux boundary condition. In general, the field solutions allow for mass in-fall towards the black hole as well as for mass outflow towards the asymptotic jet. There is strong evidence for a hollow jet structure, i.e. for a mass flow only in the outermost layers of the jet. In the asymptotic regime only the outermost 10% of the jet (in terms of radius) are likely to contain a mass flow.

The angular momentum flow and Poynting flux from the black hole into the jet could be estimated since the current distribution is known. For the field distributions investigated the solution with a concentrated jet current distribution gives angular momentum and energy losses from the hole a factor 3 higher than the other one. However, the total losses by the jet differ only slightly.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998