http://arxiv.org/abs/1702.02008
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 ($\Delta p \sim 1\%$) 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.”4) companion ($p_{R}= 7.28 \pm 0.09 \%$), 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, $p_{V} = 1.0 \pm 0.1\%$, at the polarization position angle of $\theta_{V}= -8\deg \pm 2\deg$, which is consistent in all three passbands. 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.
I. Kosenkov, A. Berdyugin, V. Piirola, et. al.
Wed, 8 Feb 17
25/65
Comments: 11 pages, 16 figures, 3 tables; Submitted to MNRAS
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