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 ∼ 15 mas from a core region, with an opening angle of 0.6c) jet during this state.