Our simultaneous three-colour ( BVR ) polarimetric observations of the low-mass black hole X-ray binary V404 Cyg show a small but statistically significant change of polarization degree ( \Delta p \sim 1 per cent ) between the outburst in June 2015 and the quiescence .
The polarization of V404 Cyg in the quiescent state agrees within the errors with that of the visually close ( 1 \farcs 4 ) companion ( p _ { R } = 7.3 \pm 0.1 per cent ) , indicating that it is predominantly of interstellar origin .
The polarization pattern of the surrounding field stars supports this conclusion .
From the observed variable polarization during the outburst we show that polarization degree of the intrinsic component peaks in the V -band , p _ { V } = 1.1 \pm 0.1 per cent , at the polarization position angle of \theta _ { V } = -7 \degr \pm 2 \degr , which is consistent in all three passbands .
We detect significant variations in the position angle of the intrinsic polarization in R band from -30 \degr to \sim 0 \degr during the outburst peak .
The observed wavelength dependence of the intrinsic polarization does not support non-thermal synchrotron emission from a jet as a plausible mechanism , but is in better agreement with the combined effect of electron ( Thomson ) scattering and absorption in a flattened plasma envelope or outflow surrounding the illuminating source .
Alternatively , the polarization signal can be produced by scattering of the disc radiation in a mildly relativistic polar outflow .
The position angle of the intrinsic polarization , nearly parallel to the jet direction ( i.e .
perpendicular to the accretion disc plane ) , is in agreement with these interpretations .