Pulsars, transients and relativistic astrophysics

Abstract:

Searches for statistical overdensities of radio sources in the NRAO VLA Sky Survey (NVSS) survey have led to the detection of high redshift clusters of galaxies (Cotter et al. 2002; Croft et al. 2001). We have carried out a similar search for overdensities using the Sydney University Molonglo Sky Survey (SUMSS). Radio observations at 20/13 cm have been made of these candidates at the Australia Telescope Compact Array (ATCA) revealing sources of diffuse cluster emission as well as groups of FR1 sources showing structures associated with clusters, indicating a cluster "hit-rate" of about 70%.

Abstract:

In this review I discuss the capabilities that the GTC, and in particular the OSIRIS spectrometer, will bring to studying massive stellar populations within Local Group galaxies and even beyond. By observing massive stars in other dwarf irregular and spiral galaxies one can probe star formation and stellar evolution in extreme environments, the wind properties of massive luminous stars, and determine distances to an accuracy of ∼10%.

Abstract:

Multifrequency observations from the VLA, VLBA and OVRO Millimeter Array of a major radio outburst of Cygnus X-3 in 2001 September are presented, measuring the evolution of the spectrum of the source over three decades in frequency, over a period of six days. Following the peak of the flare, as the intensity declines the high-frequency spectrum at frequency nu steepens from nu^{-0.4} to nu^{-0.6}, after which the spectral index remains at this latter terminal value; a trend previously observed but hitherto not satisfactorily explained. VLBA observations, for the first time, track over several days the expansion of a sequence of knots whose initial diameters are approximately 8 milliarcseconds. The light-crossing time within these plasmons is of the same order as the time-scale over which the spectrum is observed to evolve. We contend that properly accounting for light-travel time effects in and between plasmons which are initially optically thick, but which after expansion become optically thin, explains the key features of the spectral evolution, for example the observed timescale. Using the VLBA images, we have directly measured for the first time the proper motions of individual knots, analysis of which shows a two-sided jet whose axis is precessing. The best-fit jet speed is roughly beta = 0.63 and the precession period is about 5 days, significantly lower than fitted for a previous flare. Extrapolation of the positions of the knots measured by the VLBA back to zero-separation shows this to occur approximately 2.5 days after the detection of the rise in flux density of Cygnus X-3.