Pulsars, transients and relativistic astrophysics

Abstract:

We summarize HST observations of the M 87 jet, concentrating on polarimetry and spectral index maps, and compare its optical and radio structures. The evidence now supports a stratified model for the structure of the jet, in which high-energy electrons emitting optical synchrotron radiation and their lower-energy, radio-emitting counterparts occupy separate regions of the jet, with different magnetic-field configurations. The higher-energy paxticles are closer to the jet axis, where the shocks that produce the knots in the inner jet appear to originate. Knot regions have optical spectra Which axe much flatter than the average for the jet, with the flattest-spectrum regions coinciding with flux maxima of the knots. These knots are preceded by regions where perpendicular apparent magnetic fields are seen. Thus not only do we see all the necessary ingredients for in situ particle acceleration in the knots, but there is now fairly direct evidence for it as well. By tracking the changes in radio-optical and optical spectral indices in the knot regions, we can estimate the relative acceleration. and cooling time-scales in the knots.

Abstract:

We have observed a sample of Seyfert galaxies with global VLBI in order to test the predictions of the Unified Scheme for Seyferts, which hypothesises that Seyferts of type 1 and of type 2 differ only in the orientation of the axisymmetric active nucleus with respect to the observer. In this case, the parsec-scale radio structures of the two types should be similar. The 10 Seyfert 1s and 10 Seyfert 2s in our sample have been selected to have similar distributions of redshift and properties of their host galaxies: [OIII] luminosity, galaxy bulge luminosity etc. In this way, we ensure that the two subsamples of Seyferts are matched with respect to properties that axe expected to be orientation-independent. We are thus able to test the Unified Scheme rigorously. We detect all the objects that we observed and discuss the results. Our results are consistent with the prediction of the simple Unified Scheme.

Abstract:

This work presents new kinds of numerical schemes adapted to the transport and the acceleration of relativistic particles in complex multi-dimensional astrophysical flows. They use stochastic differential equations (SDE) equivalent to the diffusion-convection equation (DCE) for cosmic rays. The discretized SDE allows a time reconstruction of the particle distribution function of the relativistic particles. The method is first tested in ideal simple cases of shock acceleration with radiative losses. The ability of the different SDE schemes to solve more complex problems and their limitations are also addressed. The schemes are then applied to the transport of radio-emitting electrons in extragalactic jets. It is shown that a rescaling procedure for the diffusion coefficients can be used to compute of radiative jet structures. Finally, uses of the SDEs in conjunction with with 2D-3D (magneto-)hydrodynamical simulations are considered.

Abstract:

A summary is presented of a meeting on Particles and Fields in Radio Galaxies held in Oxford in August 2000. Recent detailed studies of radio maps and the first X-ray images from the Chandra X-ray Observatory, together with new capabilities in simulating hydromagnetic flows, are transforming our understanding of how jets are formed by accretion disks orbiting massive black holes, how these jets dissipate and how they inflate the giant radio-emitting lobes by which they were first identified. As they can be imaged in such detail, extragalactic radio sources remain central to the study of jet outflows in general although there is a strong convergence with corresponding studies of Galactic superluminal sources, gamma ray bursts and young stellar objects..

Abstract:

We present optical polarization maps of the western hot-spot of the radio galaxy Pictor A. We confirm the presence of optical emission in a bar-shaped structure extending over 24 arcsec perpendicular to the jet direction upstream of the hot-spot. We find its optical emission to be highly polarized with magnetic vectors being aligned perpendicular to the jet axis. From the high degree of polaxization we infer that the extended optical emission is of synchrotron origin. Radiative lifetimes of electrons emitting synchrotron emission at frequencies as high as 10(14) Hz are much shorter than diffusion time scales of particles accelerated within a narrow jet. Among different ways. to account for local acceleration of these electrons, we favour a scenario explaining the extended bar-shaped region as a shock-front in the back-flow of the radio-jet.