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, 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 $\sim 4~{{\ \rm per\ cent}}$) concurrently with the low inclination (∼30°) of the binary orbit.