Magnetically driven superluminal motion from rotating black holes - Solution of the magnetic wind equation in Kerr metric

We have investigated magnetically driven superluminal jets originating from rotating black holes. The stationary, general relativistic, magnetohydrody namic wind equation along collimating magnetic flux surfaces has been solve d numerically. Our jet solutions are calculated on a global scale of a spat ial range from several to several 1000 gravitational radii. Different magne tic field geometries were investigated, parameterized by the shape of the m agnetic flux surface and the magnetic flux distribution. For a given magnet ic flux surface we obtain the complete set of physical parameters for the j et flow. In particular, we apply our results to the Galactic superluminal s ources GRS 1915+105 and GRO 1655-40. Motivated by the huge size indicated f or the Galactic superluminal knots of about 10(9) Schwarzschild radii. we p oint out the possibility that the jet collimation process in these sources may be less efficient and therefore intrinsically different to the AGN. Our results show that the observed speed of more than 0.9 c can be achieved in general by magnetohydrodynamic acceleration. The velocity distribution alo ng the magnetic field has a saturating profile. The asymptotic jet velocity depends either on the plasma magnetization (for a fixed field structure) o f on the magnetic flux distribution (for fixed magnetization). The distance where the asymptotic velocity is reached, is below the observational resol ution for GRS 1915+105 by several orders of magnitude. Further, we find tha t highly relativistic speeds can be reached also for jets not emerging from a region close to the black hole: if the flow magnetization is sufficientl y large. The plasma temperature rapidly decreases from about 10(10) K at th e foot point of the jet to about 10(6) K at a distance of 5000 gravitationa l radii from the source. Temperature and the mass density follow a power la w distribution with the radius. The jet magnetic field is dominated hy the toroidal component, whereas the velocity field is dominated by the poloidal component.