Operational since 2002 on-board the INTEGRAL observatory , the SPI spectrometer can be used to perform polarization measurements in the hard X-ray/soft \gamma -ray domain ( \sim 130 keV - 8 MeV ) .
However , this phenomenon is complex to measure at high energy and requires high fluxes .
Cyg X-1 appears as the best candidate amongst the X-ray binaries since it is one of the brightest persistent sources in this energy domain .
Furthermore , a polarized component has recently been reported above 400 keV from IBIS data .
We have therefore dedicated our efforts to develop the required tools to study the polarization in the INTEGRAL SPI data and have first applied them to 2.6 Ms of Cyg X-1 observations , covering 6.5 years of the INTEGRAL mission .

We have found that the high energy emission of Cyg X-1 is indeed polarized , with a mean polarization fraction of 76 \% \pm 15 \% at a position angle estimated to 42 ^ { \circ } \pm 3 ^ { \circ } , for energies above 230 keV .
The polarization fraction clearly increases with energy .
In the 130-230 keV band , the polarization fraction is lower than 20 \% , but exceeds 75 \% between 370 and 850 keV , with the ( total ) emission vanishing above this energy .
This result strongly suggests that the emission originates from the jet structure known to emit in the radio domain .
The same synchrotron process could be responsible for the emission from radio to MeV , implying the presence of high energy electrons .
This illustrates why the polarization of the high energy emission in compact objects is an increasingly important observational objective .