A relativistic outflow model of the X-ray polarization in Cyg X-1

We propose that the polarization of the emission from the X-ray binary Cygnus X-1, measured using the Imaging X-ray Polarimetry Explorer (IXPE), is imprinted by bulk Comptonization of coronal emission in a mildly relativistic wind or jet with a hollow-cone geometry. Models based on scattering in a static corona overlying a thin accretion disc have difficulty reproducing the relatively high polarization degree (PD ~4%) concurrently with the low inclination (~30 deg) of the binary orbit. We show that bulk outflow with a Lorentz factor > 1.5 is adequate to reproduce the observed PD, with position angle parallel to the large-scale jet, provided that the scattering occurs in a conical sheath offset from the jet axis and our line of sight aligns roughly with the opening angle of the cone. Physically, this flow geometry could represent the entrainment of dense material near the base of an accelerating jet as it passes through the disc corona, or a slow (but still relativistic) sheath around a fast jet. If similar outflows are present in other X-ray binaries at higher inclination, we might expect to see still higher degrees of linear polarization ~10% with an orientation perpendicular to the jet direction.