VLBA Observations of the Superluminal Radio Jet from Cygnus X-1 in the Low/Hard X-Ray State

Abstract

We present the detection of a resolved jet-like radio-emitting structure from the black-hole candidate and X-ray binary source Cygnus X-l at both 8.4 GHz and 15 GHz. The structure in the milli-arcsecond resolution observation with the VLBA at 15 GHz shows an extended emission region to the north-west and possibly also perpendicular emitting regions, believed to be in the orbital plane. This X-ray state is believed to be associated with an advective accretion environment. These observations were spaced 2 days apart over the 5.6-day orbital period. The images confirm the existence of persistent emission from the source from both an unresolved core and a variable, and at least moderately (> 0.6c) relativistic jet, during the low/hard X-ray state. However, at this resolution and wavelength they do not show any sign of the emission in the orbital plane observed at a 5σ level at 15 GHz in the initial epoch.