Katherine Blundell OBE

Abstract:

With particular reference to the BL Lac object, Markarian 501, we emphasize the importance of X-ray and gamma-ray observations for elucidating the parameters of parsec scale and sub-parsec.scale jets. In particular the magnetic field, particle energy density and Doppler factor can be inferred from simultaneous observations. The energy flux in the Mkn 501 jet is particle dominated. If the composition is electron-proton then the minimum electron Lorentz factor, gamma(1) similar to 100. Consideration of a sample of quasar and BL Lac jets, shows that these jets may consist of electron-positron pairs, also if gamma(1) similar to 100. Seyfert jets may be examples of jets in which the plasma composition is predominantly electron-proton, suggesting that they originate at larger radii (with respect to the gravitational radius) than jets in radio-loud galaxies.

Abstract:

Considerable evidence has accumulated that strongly favours interstellar scintillation (ISS) as the principal mechanism causing intra-day variability (IDV) at cm wavelengths. This includes the observed frequency dependence of IDV, the measured time-delay in the IDV pattern as seen at two widely spaced radio telescopes, and the presence of an annual time-signature in the behaviour of the long-term IDV variability. While ISS reduces the implied brightness temperatures, they remain uncomfortably high. There appear to be several obstacles to explaining the observations with synchrotron theory: first, the brightness temperatures appear to be up to two orders of magnitude above the inverse Compton limit, second, the observed IDV lifetimes of more than a decade are much longer than expected, and third, there is the presence of strong and variable circular polarization in several sources.

Abstract:

We present the results of numerical simulations of the buoyant rise of the cocoons of radio galaxies after they come into pressure equilibrium with the surrounding gas. As in the later stages of nuclear explosions, they evolve into rising tori which can uplift dense, cold gas from the cluster centre. We also calculate the expected radio emission from these structures and suggest that this may explain the 'ears' observed in the large scale radio structure of M 87. Finally, the plasma will reach its terminal height and start to spread out. The resulting 'pancakes' may explain the circular regions of diffuse emission in M 87.

Abstract:

We have observed 17 sources in a complete sample of FRIs in nearby UGC galaxies with the VLBA at 1.67 GHz. We detect core-jets in 60% and twin-jets in 10%. The data (alignment, sidedness ratios, detection statistics) are consistent with relativistic Doppler boosting effects. The luminosity per unit length of the jets fades with distance l from the core as L-j proportional to l(-2). The fading is not consistent with either deceleration of the jets (with subsequent reduction of Doppler boosting) or synchrotron losses. However, the fading can be explained by adiabatic losses in a constant velocity jet with constant opening angle and magnetic field perpendicular to the jet axis. If confirmed by VLBA polarimetry, this would imply that the jets must decelerate on scales larger than tens of parsec.

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.