Pulsars, transients and relativistic astrophysics

Abstract:

We discuss the various contributions to the jet luminosity in proton blazar models of active galactic nuclei and describe a method of estimating the jet luminosity from the observed spectral energy distribution (SED) and the fitted model parameters. We apply this to a synchrotron proton blazar (SPB) model for Markarian 501.

Abstract:

The parsec-scale radio jets of BL Lacertae objects imaged with Very Long Baseline Interferometry (VLBI) show a number of distinctive features. The most striking of these is the dominance of transverse magnetic fields on the wide range of scales probed by VLBI observations at frequencies from 43 to 5 GHz. Some compact VLBI components with transverse fields axe undoubtedly relativistic shocks, in which the transverse field has been enhanced by compression. However, there is considerable evidence that we axe also detecting the toroidal component of an underlying helical magnetic field associated with the VLBI jet.

Abstract:

In February 1997, we obtained multi-frequency polarization VLBA observations of 23 sources from a complete sample of northern BL Lac objects. Total intensity and linear polarization images of two sources (0745+241 and 0820+225) showing evidence for the presence of sheaths of longitudinal magnetic field surrounding their jets are presented. These results suggest that appreciable amounts of thermal plasma are often present on parsec scales, and that the jet magnetic field can be influenced by interaction with this surrounding medium.

Abstract:

FRIIb radio sources axe a special subset of all classical double radio sources with particularly simple radio bridge structures. At low radio frequencies, the surface brightness across the ridge-line of the source is accurately reproduced by accounting for adiabatic expansion in the lateral direction by an amount indicated by the observed change in the width of the radio bridge as a function of position. Thus, the physics of FRIIb sources is quite simple. The radio properties of an FRIIb source may provide a useful probe of the pressure, density, and temperature of gas surrounding the radio source. Estimates of the ambient gas density obtained from considerations of the rain-pressure confinement of the forward region of FRIIb radio sources axe discussed here. Typical ambient gas densities and the composite density profile indicate that the sources lie in gaseous environments very much like the cores of galaxy clusters. Some evolution of the core gas density and radius with redshift axe suggested by the data.

Abstract:

The presence of optical synchrotron jets in radio galaxies is relatively unusual. We show that of the nearest five FRI 3CR radio galaxies showing optical jets, four display evidence for almost circular, presumably face-on, dust discs. None of the other twenty nearby FRIs in our sample show circular dust discs, although dust is found in 19/20 cases. This is strong support for the two-fold idea that (1) jets emerge close to perpendicular to inner gas discs and (2) optical non-thermal synchrotron emission is seen only when the jet points towards the observer. The implied critical angle to the line-of-sight is approximately 30-40 degrees, i.e. if the angle of the jet to the line-of-sight is less than about 35 degrees we see an optical jet. The corresponding Lorentz factor is gamma approximate to 1.5, which is consistent with current observations of jet proper motion that show an apparent velocity range from; approximate to 0.6c to 6c for M 87 in the optical (Biretta, Sparks & Macchetto 1999). The relatively low speeds implied by the dust discs may be due to a global deceleration of the jet as in unified theories, or else to stratification within the jet. Unresolved nuclei are common in the optical. Their luminosities are also consistent with the beaming concept when compared to inclination inferred from the dust lanes. The disc sizes are typically several hundred parsecs, to kiloparsec size. The galaxy with an optical jet that does not show a face-on disc, M 87, instead has more complex radial dust and ionized gas filaments.