BeyondPlanck XV. Polarized foreground emission between 30 and 70 GHz

We constrain polarized foreground emission between 30 and 70 GHz with the Planck Low Frequency Instrument (LFI) WMAP data within global Bayesian BeyondPlanck framework. combine for first time full-resolution LFI time-ordered low-resolution sky maps at 33, 40 61 GHz. Spectral parameters are fit a likelihood defined native resolution of each frequency channel. This analysis represents implementation true multi-resolution component separation applied to CMB observations both amplitude spectral energy distribution (SED) parameters. For synchrotron emission, we approximate SED as power-law in find that low signal-to-noise ratio current strongly limits number free may be robustly constrained. partition into four large disjoint regions (High Latitude; Galactic Spur; Plane; Center), associated its own index. High Latitude region is prior-dominated, while Center contaminated by residual instrumental systematics. The two remaining appear signal-dominated, these derive indices $\beta_{\mathrm s}^{\mathrm{Spur}}=-3.17\pm0.06$ s}^{\mathrm{Plane}}=-3.03\pm0.07$, good agreement previous results. thermal dust assume modified blackbody model single index across full sky. $\beta_{\mathrm{d}}=1.64\pm0.03$, which slightly steeper than reported from HFI data, but still statistically consistent 2$\sigma$ confidence level.