Abstract. -In a companion paper, a mathematical formalism to describe the polarimetric response of a radio interferometer was presented. Some of the instrumental parameters, however, are either unknown or poorly known. Here we consider the determination of these parameters both by a traditional radio-interferometry instrumental approach as well as by using optical polarimetry principles. In doing so, we establish links between the two fields. We show that some degrees of freedom cannot be solved for with various calibration or self-calibration schemes. These degrees of freedom are identified with instrumental parameters and physical source properties. The number of unsolvable degrees of freedom is reduced for a long synthesis with alt-az antennas. We also consider the effect of errors in the assumed instrumental parameters on the resultant calibrated data. The polarimetric calibration procedure for some telescopes is reviewed in the context of this analysis.

We report on radio observations of the 1999 September event of the X-ray transient V4641 Sgr (\XTE J1819[254\SAX J1819.3[2525). This event was extremely rapid in its rise and decay across radio, optical, and X-ray wavelengths ; the X-rays rose to 12 crab within 8 hr and faded to below 0.1 crab in less than 2 hr. Radio observations were made with seven telescopes during the Ðrst day following the onset of the strong X-ray event, revealing a strong radio source that was detected for 3 further weeks by the more sensitive telescopes. The radio source was resolved even in the Ðrst Very Large Array (VLA) images (September 16.027 UT), being long with an axis ratio of at least 10 : 1. The total Ñux D0A .25 density decayed by a factor of D4 over the Ðrst day, and by September 17.94 UT the radio emission was conÐned to a slowly decaying, marginally resolved remnant located at one side of the early elongated emission. The H I absorption spectrum gives a minimum kinematic distance of about 400 pc ; various other arguments suggest that the true distance is not much greater than this. The inferred proper motions for the early extended emission day~1) correspond to v/c D 1.0È (0A .4È1A .1 3.2 (d/0.5 kpc), and this together with the radio morphology argues that this is a relativistic jet source like GRS 1915]105 and GRO J1655[40. The proper motion of the late-time remnant is at least 100 times smaller. One simple interpretation posits the ejection of a single short-lived jet segment, followed by a more slowly decaying, optically thin jet segment ejection. These two components can explain both the multifrequency radio light curves and the radio images. The most likely parameters for the fast-jet system with net-averaged proper motion of day~1, assuming d \ 0.5 kpc, are v D 0.85c and D0A .4 i D 63¡, where i is the inclination to the line of sight. The corresponding apparent velocities are 1.4c and 0.6c for the approaching and receding jets, making V4641 Sgr the closest superluminal jet source known.

We report multifrequency radio observations of GRO J1655[40 obtained with the Australia Telescope Compact Array, the Molonglo Observatory Synthesis Telescope and the Hartebeesthoek Radio Astronomy Observatory at the time of the major hard X-ray and radio outbursts in 1994 AugustÈ September. The radio emission reached levels of the order of a few Jy and was found to be linearly polarized by up to 10%, indicating a synchrotron origin. The light curves are in good agreement with those measured with the VLA, but our closer time sampling has revealed two new short-lived events and signiÐcant deviations from a simple exponential decay. The polarization data show that the magnetic Ðeld is well ordered and aligned at right angles to the radio jets for most of the monitoring period. The time evolution of the polarization cannot be explained solely in terms of a simple synchrotron bubble model, and we invoke a hybrid "" core-lobe ÏÏ model with a core which contributes both synchrotron and free-free emission and "" lobes,ÏÏ which are classical synchrotron emitters.