Evidence for a black hole spin--orbit misalignment in the X-ray binary Cyg X-1

Cyg X-1 is probably the most studied and best understood black-hole X-ray binary. Recently, its accretion geometry has been probed with the X-ray polarization. The position angle of the X-ray emitting flow was found to be aligned with the position angle of the radio jet in the plane of the sky. At the same time, the observed high polarization degree could be obtained only for a high inclination of the X-ray emitting flow, indicating a misalignment between the binary axis and the black hole spin. The jet, in turn, is believed to be directed by the spin axis, hence similar misalignment is expected between the jet and binary axes. We test this hypothesis using very long (up to about 26 years) multi-band radio observations. We find the misalignment of $20^\circ$-$30^\circ$; however, on the contrary to the earlier expectations, the misalignment lies in the plane of the sky and not along the line of sight. Furthermore, the presence of the misalignment questions our understanding of the evolution of this binary system.