Global Jet Watch

Abstract:

We present new results on magnetized accretion discs launching bipolar, self-collimated jets. We show that the jet mass-loading depends critically on the disc vertical equilibrium. In particular, it is strongly influenced by any entropy generation occurring at the disc surface. Discs around supermassive black holes and close to a magnetohydrostatic balance have the potential to produce mildly relativistic jets.

Abstract:

Analytical relations between the mean jet speed and the mean jet flux asymmetry and between their variances are derived in the context of a unified scheme. The mean jet speeds of classical double radio sources axe estimated by using the asymmetry of jet-counter-Jet flux densities taken from the 3CRR and B2 samples. For FRI radio sources the mean jet speed is similar to 0.54 +/- 0.03c, while for FRII low-redshift radio galaxies and intermediate-redshift quasars the values found are similar to 0.4c +/- 0.06c and greater than or equal to 0.6c respectively.

Abstract:

The quasar PKS 0405-385 exhibits episode's of unusually strong hourly variability at 8.6, 4.8, 2.4 and 1.4 GHz. These variations, seen first in June 1996, ceased after several weeks, but reappeared in November 1998 and were monitored closely at the. ATCA. We argue that our data are in agreement with interstellar scintillation of a source component which is a few micro-arcseconds in size. This claim is further supported by our recent finding of the time delay in the variability pattern measured between the ATCA and the VLA.

Abstract:

We extend the model of Blandford & Payne (1982) for magnetocentrifugally driven winds by presenting solutions that satisfy the singularity conditions at all critical surfaces. In these solutions, the asymptotic supercritical zone is causally disconnected from the upstream region,of the outflow, unlike the,situation in previous studies.

Abstract:

I will discuss several issues related to the acceleration and collimation of jets from AGNs. Hydromagnetic stresses provide the best bet both for accelerating relativistic flows And for providing a certain amount of initial collimation. However, there are limits to how much self-collimation can be achieved without the help of an external pressurised medium. Moreover, existing models which postulate highly organized poloidal magnetic flux over much of the flow may be unrealistic. Instead, a large fraction of the magnetic energy may reside in a highly disorganized "chaotic" field. Such a field can also accelerate the flow to relativistic speeds, in some cases with greater efficiency than highly organized fields, but at the expense of self-collimation. In any case, acceleration to highly relativistic speeds may be a gradual process, occurring over several decades in radius.