Magnetically driven superluminal motion from rotating black holes

Solution of the magnetic wind equation in Kerr metric

Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Corresponding author: C. Fendt, cfendt@aip.de

Received: 21 August 2000
Accepted: 19 January 2001

Abstract

We have investigated magnetically driven superluminal jets originating from rotating black holes. The stationary, general relativistic, magnetohydrodynamic wind equation along collimating magnetic flux surfaces has been solved numerically. Our jet solutions are calculated on a global scale of a spatial range from several to several 1000 gravitational radii. Different magnetic field geometries were investigated, parameterized by the shape of the magnetic flux surface and the magnetic flux distribution. For a given magnetic flux surface we obtain the complete set of physical parameters for the jet flow. In particular, we apply our results to the Galactic superluminal sources GRS 1915+105 and GRO 1655-40. Motivated by the huge size indicated for the Galactic superluminal knots of about 10⁹ Schwarzschild radii, we point 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 along the magnetic field has a saturating profile. The asymptotic jet velocity depends either on the plasma magnetization (for a fixed field structure) or on the magnetic flux distribution (for fixed magnetization). The distance where the asymptotic velocity is reached, is below the observational resolution for GRS 1915+105 by several orders of magnitude. Further, we find that highly relativistic speeds can be reached also for jets not emerging from a region close to the black hole, if the flow magnetization is sufficiently large. The plasma temperature rapidly decreases from about 10¹⁰ K at the foot point of the jet to about 10⁶ K at a distance of 5000 gravitational radii from the source. Temperature and the mass density follow a power law distribution with the radius. The jet magnetic field is dominated by the toroidal component, whereas the velocity field is dominated by the poloidal component.