2007ApJS..170..335P

The Wilkinson Microwave Anisotropy Probe (WMAP) has mapped the entire sky in five frequency bands between 23 and 94 GHz with polarization-sensitive radiometers. We present 3 year full-sky maps of the polarization and analyze them for foreground emission and cosmological implications. These observations open up a new window for understanding how the universe began and help set a foundation for future observations. WMAP observes significant levels of polarized foreground emission due to both Galactic synchrotron radiation and thermal dust emission. Synchrotron radiation is the dominant signal at l<50 and ν≲40 GHz, while thermal dust emission is evident at 94 GHz. The least contaminated channel is at 61 GHz. We present a model of polarized foreground emission that captures the large angular scale characteristics of the microwave sky. After applying a Galactic mask that cuts 25.7% of the sky, we show that the high Galactic latitude rms polarized foreground emission, averaged over l=4-6, ranges from ~5 µK at 22 GHz to ≲0.6 µK at 61 GHz. By comparison, the levels of intrinsic CMB polarization for a ΛCDM model with an optical depth of τ=0.09 and assumed tensor-to-scalar ratio r=0.3 are ~0.3 µK for E-mode polarization and ~0.1 µK for B-mode polarization. To analyze the maps for CMB polarization at l<16, we subtract a model of the foreground emission that is based primarily on a scaling WMAP's 23 GHz map. In the foreground-corrected maps, we detect l(l+1)C^EE_l=2–6/2π=0.086±0.029 (µK)². This is interpreted as the result of rescattering of the CMB by free electrons released during reionization at z_r=10.9^+2.7_–2.3 for a model with instantaneous reionization. By computing the likelihood of just the EE data as a function of τ we find τ=0.10±0.03. When the same EE data are used in the full six-parameter fit to all WMAP data (TT, TE, EE), we find τ=0.09±0.03. Marginalization over the foreground subtraction affects this value by δτ<0.01. We see no evidence for B modes, limiting them to l(l+1)C^BB_l=2–6/2π=-0.04±0.03 (µK)². We perform a template fit to the E-mode and B-mode data with an approximate model for the tensor scalar ratio. We find that the limit from the polarization signals alone is r<2.2 (95% CL), where r is evaluated at k=0.002/Mpc. This corresponds to a limit on the cosmic density of gravitational waves of Ω_GWh²<5x10^–12. From the full WMAP analysis, we find r<0.55 (95% CL) corresponding to a limit of Ω_GWh²<1x10^–12 (95% CL). The limit on r is approaching the upper bound of predictions for some of the simplest models of inflation, r∼0.3.