2013MNRAS.435L..48S

The ejection of a relativistic jet has been observed in the luminous Galactic low-mass X-ray binary Cygnus X-2. Using high-resolution radio observations, a directly resolved ejection event has been discovered while the source was on the horizontal branch of the Z-track. Contemporaneous radio and X-ray observations were made with the European VLBI Network at 6 cm and the Swift X-ray observatory in the 0.3-10 keV band. This has been difficult to achieve because of the previous inability to predict jet formation. Two sets of ∼ 10 h observations were spaced 12 h apart, the jet apparently switching on during Day 1. The radio results show an unresolved core evolving into an extended jet. A preliminary value of jet velocity v/c of 0.33±0.12 was obtained, consistent with previous determinations in Galactic sources. Simultaneous radio and X-ray lightcurves are presented and the X-ray hardness ratio shows the source to be on the horizontal branch where jets are expected. The observations support our proposal that jet formation can in future be predicted based on X-ray intensity increases beyond a critical value.