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 Very Long Baseline Array (VLBA) at 15.4 GHz. Later observations with the VLBA [and including the phased-up, Very Large Array (VLA)] at 8.4 GHz show an extended jet-like feature extending to similar 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 (> 0.6c) jet during this state.