Relativistic outflows from radio emitting X-ray binaries

Only a small fraction of the known X-ray binaries have radio emission and it is unclear how the presence of radio emission is related to their other physical properties. Some of these objects show well collimated jets and have been called micro-quasars. High velocity outflows at around 0.3c were discovered from the famous X-ray binaries SS433 and Cygnus X-3 in the early 1980s. However more recent observations have discovered XRBs showing superluminal expansion corresponding to intrinsic jet velocities of around 0.9c. Observations with the VLBA of Cygnus X-3 taken in 1995 show that the object underwent apparent superluminal expansion and contraction when in a state of rapid flaring, and suggest that outflow in the form of collimated jets is not the only mode of ejection which can take place in these sources.