We revisit the discovery outburst of the X-ray transient XTE J1550 - 564 during which relativistic jets were observed in 1998 September , and review the radio images obtained with the Australian Long Baseline Array , and lightcurves obtained with the Molonglo Observatory Synthesis Telescope and the Australia Telescope Compact Array .
Based on H i spectra , we constrain the source distance to between 3.3 and 4.9 kpc .
The radio images , taken some two days apart , show the evolution of an ejection event .
The apparent separation velocity of the two outermost ejecta is at least 1.3 c and may be as large as 1.9 c ; when relativistic effects are taken into account , the inferred true velocity is \geq 0.8 c .
The flux densities appear to peak simultaneously during the outburst , with a rather flat ( although still optically thin ) spectral index of -0.2 .