A novel CMB polarization likelihood package for large angular scales built from combined WMAP and Planck LFI legacy maps

We present a CMB large-scale polarization dataset obtained by combining WMAP Ka, Q and V with Planck 70 GHz maps. We employ the legacy frequency maps released by the WMAP and Planck collaborations and perform our own Galactic foreground mitigation technique, which relies on Planck 353 GHz for polarized dust and on Planck 30 GHz and WMAP K for polarized synchrotron. We derive a single, optimally-noise-weighted, low-residual-foreground map and the accompanying noise covariance matrix. These are shown, through $chi^2$ analysis, to be robust over an ample collection of Galactic masks. We use this dataset, along with the Planck legacy Commander temperature solution, to build a pixel-based low-resolution CMB likelihood package, whose robustness we test extensively with the aid of simulations, finding excellent consistency. Using this likelihood package alone, we constrain the optical depth to reionazation $tau=0.069^{+0.011}_{-0.012}$ at $68%$ C.L., on 54% of the sky. Adding the Planck high-$ell$ temperature and polarization legacy likelihood, the Planck lensing likelihood and BAO observations we find $tau=0.0714_{-0.0096}^{+0.0087}$ in a full $Lambda$CDM exploration. The latter bounds are slightly less constraining than those obtained employing Planck HFI CMB data for large angle polarization, that only include EE correlations. Our bounds are based on a largely independent dataset that does include also TE correlations. They are generally well compatible with Planck HFI preferring slightly higher values of $tau$. We make the low-resolution Planck and WMAP joint dataset publicly available along with the accompanying likelihood code.