A relativistic jet from Cygnus X-1 in the low/hard X-ray state

We present the detection of a radio-emitting jet from the black-hole candidate and X-ray binary source Cygnus X-1. Evidence of a bright core with a slightly extended structure was found on milliarcsecond resolution observations with the VLBA at 15.4 GHz. Later observations with the VLBA (and including the phased up VLA) at 8.4 GHz show an extended jet-like feature extending to ∼ 15 mas from a core region, with an opening angle of < 2◦. In addition, lower resolution MERLIN observations at 5 GHz show that the source has < 10 per cent linear polarization. The source was in the low/hard X-ray state during the observations, and the results confirm the existence of persistent radio emission from an unresolved core and a variable relativistic (> 0.6 c) jet during this state.