X - ray states and radio emission in the black hole candidate XTE J 1550 − 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 have been – 2 – conducted with the Australia Telescope Compact Array (ATCA) and have allowed us to sample the radio behavior of XTE J1550−564 in the X-ray Low Hard and Intermedi-ate/Very High states. We observed optically thin radio emission from XTE J1550−564 five days after a transition to an Intermediate/Very High state, but we observed no radio emission six 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 significant fraction of the total luminosity of the system. We suggest that the optically thin synchrotron synchrotron radio emission detected five 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 outflow. We discuss the properties of radio emission in the X-ray states of BHCs.