We report on radio and X-ray observations of the black hole candidate (BHC) XTE J1550[564 performed during its 2000 X-ray outburst. Observations were conducted with the Australia Telescope Compact Array and allowed us to sample the radio behavior of XTE J1550[564 in the X-ray low hard and intermediate/very high states. We observed optically thin radio emission from XTE J1550[564 5 days after a transition to an intermediate/very high state, but we observed no radio emission 6 days later, while XTE J1550[564 was still in the intermediate/very high state. In the low hard state, XTE J1550[564 is detected with an inverted radio spectrum. The radio emission in the low hard state most likely originates from a compact jet ; optical observations suggest that the synchrotron emission from this jet may extend up to the optical range. The total power of the compact jet might therefore be a signiÐcant fraction of the total luminosity of the system. We suggest that the optically thin radio emission detected 5 days after the transition to the intermediate/very high state is due to a discrete ejection of relativistic plasma during the state transition. Subsequent to the decay of the optically thin radio emission associated with the state transition, it seems that in the intermediate/very high state the radio emission is quenched by a factor greater than 50, implying a suppression of the outÑow. We discuss the properties of radio emission in the X-ray states of BHCs.

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 the first radio interferometric search at 843 MHz for fast transients, particularly Fast Radio Bursts (FRBs). The recently recommissioned Swinburne University of Technology's digital backend for the Molonglo Observatory Synthesis Telescope array (the UTMOST) with its large collecting area (18,000 m 2 ) and wide instantaneous field of view (7.80 deg 2 ) is expected to be an efficient tool to detect FRBs. As an interferometer it will be capable of discerning whether the FRBs are truly a celestial population. We show that UTMOST at full design sensitivity can detect an event approximately every few days. We report on 2 preliminary FRB surveys at about 7% and 14% respectively of the array's final sensitivity. Several pulsars have been detected via single pulses and no FRBs were discovered with pulse widths (W ), in the range 655.36 µs < W < 41.9 ms and dispersion measures (DMs) in the range 100 −3.2.

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.