A polarized synchrotron template for cosmic microwave background polarization experiments based on WMAP data

We build template maps for the polarized Galactic synchrotron emission on large angular scales (7degrees full width at half maximum), in the 20-90 GHz microwave range, by using Wilkinson Microwave Anisotropy Probe (WMAP) data. The method, presented in recent work, requires a synchrotron total-intensity survey and the polarization horizon to model the polarized intensity and a starlight polarization map to model polarization angles. The basic template is obtained directly at 23 GHz with about 94 per cent sky coverage by using the synchrotron map released by the WMAP team. Extrapolations to 32, 60 and 90 GHz are performed by computing a synchrotron spectral-index map, which strongly reduces previous uncertainties in passing from low (1.4 GHz) to microwave frequencies. Differing from low-frequency data, none of our templates present relevant structures out of the Galactic plane. Our map at 90 GHz suggests that the synchrotron emission at high Galactic latitudes is low enough to allow robust detection of the E-mode component of the cosmological signal on large scale, even in models with low reionization (tau= 0.05). Detection of the weaker B-mode on the largest scales (l < 10) might be jeopardized unless the value tau= 0.17 found by WMAP is confirmed, and tensor-to-scalar fluctuation power ratio T/S > 0.1. For lower levels of the gravitational-wave background the B-mode seems to be accessible only at the lsimilar to 100 peak and in selected low-synchrotron emission areas.