Composition, collimation, contamination: The jet of Cygnus X-1

We model the observed size and brightness of the VLBA radio core of the jet in Cygnus X-1 to derive an expression for the jet power as a function of basic jet parameters. We apply this expression to recent constraints on the jet power from observations of a large-scale shocked shell around the source by Gallo and coworkers, which leads us to a set of alternative conclusions: either (1) the jet contains large amounts of protons (>= 2000 protons per radio- emitting electron), (2) it has a very low volume filling factor of f <= 3 x 10(-5), (3) the steady, radio-emitting VLBA jet is not the source of the kinetic energy powering the ISM shell, or ( 4) its asymptotic behavior differs fundamentally from a broad set of plausible analytic jet models.