X-ray states and radio emission in the black hole candidate XTE J1550-564

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 samp le the radio behavior of XTE J1550-564 in the X-ray low hard and intermedia te/very high states. We observed optically thin radio emission from XTE J15 50-564 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 d etected with an inverted radio spectrum. The radio emission in the low hard state most likely originates from a compact jet; optical observations sugg est that the synchrotron emission from this jet may extend up to the optica l range. The total power of the compact jet might therefore be a significan t fraction of the total luminosity of the system. We suggest that the optic ally thin radio emission detected 5 days after the transition to the interm ediate/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 gre ater than 50, implying a suppression of the outflow. We discuss the propert ies of radio emission in the X-ray states of BHCs.