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Appendix A: U_φ image and S/N map

Figure A.1 presents the U_φ images. Since the scattered light from a circumstellar disk is expected to be linearly polarized in the azimuthal direction under the assumption of single scattering, U_φ contains no signal, but only noise of the same magnitude as the noise in the Q_φ image. For multiple scattering, the assumption of polarization only in azimuthal direction still holds approximately, especially for close to face-on disks, and the deviations from azimuthal polarizations are expected to be well below the noise level (Avenhaus et al. 2014).

Figure A.2 provides qualitative S/N maps. They were obtained by first smoothing both Q_φ and U_φ with a Gaussian kernel of 37 mas (close to the instrument PSF) and then dividing

the Q_φ by the standard deviation in the U_φ images. The standard deviation in the U_φ image was calculated over a Gaussian-weighted area centered on the respective image point with a FWHM of 260 mas. We note that these do not strictly provide S/N estimates. Indeed, the noise reported in these maps depends on the area considered for the calculation of the variance from the U_φ images. Moreover, the first step smoothes out the readout noise and other small-scale (smaller than the smoothing kernel) noise (but not large-scale systematic noise). These maps are still very useful, as a feature detected in the Q_φ image and appearing at a location of the map that has a high S/N value, should be considered as real. An example is feature 4, which lies in an area of high S/N and can thus be considered real, even though it does not seem apparent from the S/N maps alone.

	Fig. A.1 Uφ images (left, December 2014; right, March 2015). East is toward the left. Each pixel has been scaled with the square of its distance from the star, r2, to compensate for the r-2 dependency of the stellar illumination. The color scale is arbitrary.
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	Fig. A.2 S/N maps with contours every 3σ (left, December 2014; right, March 2015).
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Appendix B: Probability plot

Figure B.1 shows the 2D probability distributions for each pair of free parameters.

	Fig. B.1 Bayesian probabilities for the simultaneous fit of the two spirals. The top panel of each column provides the integrated 1D probability for each parameter. The lower panels in each column are the 2D probability maps for each pair of parameters.
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© ESO, 2015