  The accretion of matter by compact objects can be inhibited by radiation
pressure if the luminosity exceeds the critical value, known as the Eddington
limit. Discovery of ultraluminous X-ray sources has shown that accretion can
proceed even when the apparent luminosity significantly exceeds this limit.
High apparent luminosity might be produced thanks to geometric beaming of the
radiation by an outflow. The outflow half-opening angle, which determines the
amplification due to beaming, has never been robustly constrained. Using the
Imaging X-ray Polarimetry Explorer, we made the measurement of X-ray
polarization in the Galactic X-ray binary Cyg X-3. We find high, over 20%,
nearly energy-independent linear polarization, orthogonal to the direction of
the radio ejections. These properties unambiguously indicate the presence of a
collimating outflow in the X-ray binary Cyg~X-3 and constrain its half-opening
angle, <15 degrees. Thus, the source can be used as a laboratory for studying
the super-critical accretion regime. This finding underscores the importance of
X-ray polarimetry in advancing our understanding of accreting sources.
