Pulsars, transients and relativistic astrophysics

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.

Abstract:

If magnetic fields are near the energy equipartition value in the lobes of FR II radio sources, it can be shown that it is very unlikely that such fields are passively advected outward with the jet that creates the source. This has led to the idea that the fields are amplified within the lobes; and a likely site for this is the complex and turbulent region inside the hot-spots, as suggested by Blundell, Rawlings & Willott (1999). if this process occurs, a natural question is the subsequent evolution of the field strength and geometry as the field and fluid leave the hot-spot and fill the lobe. This evolution is followed via a time dependent turbulent MHD calculation under a variety of conditions. While the small scale field can decay away rather quickly, the large scale field structures remain surprisingly robust.

Abstract:

Electromagnetic radiation from jets is normally considered to be incoherent synchrotron emission. Some aspects of radiation from both stellar and extragalactic jets could, however, be due to coherent emission mechanisms. Coherent radiation mechanisms have the advantage of being able to account for the high brightness temperatures inferred from some observations. One of the most popular coherent radio emission mechanisms is the electron cyclotron maser instability. In this article we discuss electron cyclotron maser emission associated with particular types of charged particle acceleration commonly inferred in astrophysical jets, including turbulence and shocks.

Abstract:

We model the brightness and polarization structure of the inner jets in the low-luminosity radio galaxy 3C 31 on the assumption that they are intrinsically symmetrical, axisymmetric relativistic jets. Our approach is to make parametrized models of velocity, emissivity and field ordering, to predict the radio emission and to optimize the parameters by fitting to deep, high-resolution VLA observations of Stokes I, Q and U. Our models axe in excellent agreement with the observations for an angle to the line of sight of approximate to 50degrees. The jets decelerate from v/c approximate to 0.9 to sub-relativistic speeds on several-kiloparsec scales. We use this velocity variation, together with the formulation of conservation of particle number, energy and momentum given by Bicknell (1994), to calculate the physical parameters of the flow. ROSAT observations constrain the external pressure distribution, allowing us to derive unique solutions for pressure, density, Mach number and mass flux as functions of distance from the nucleus. Both stellar mass injection and entrainment of the surrounding IGM axe likely to contribute to jet deceleration.