SPATIAL VARIATIONS IN THE SPECTRAL INDEX OF POLARIZED SYNCHROTRON EMISSION IN THE 9 yr WMAP SKY MAPS

We estimate the spectral index, beta, of polarized synchrotron emission as observed in the 9 yr Wilkinson Microwave Anisotropy Probe sky maps using two methods, linear regression (T-T plot) and maximum likelihood. We partition the sky into 24 disjoint sky regions and evaluate the spectral index for all polarization angles between 0. and 85 degrees in steps of 5 degrees. Averaging over polarization angles, we derive a mean spectral index of beta(all-sky) = -2.99 +/- 0.01 in the frequency range of 23-33 GHz. We find that the synchrotron spectral index steepens by 0.14 from low to high Galactic latitudes, in agreement with previous studies, with mean spectral indices of beta(plane) = -2.98 +/- 0.01 and beta(high-lat) = -3.12 +/- 0.04. In addition, we find a significant longitudinal variation along the Galactic plane with a steeper spectral index toward the Galactic center and anticenter than toward the Galactic spiral arms. This can be well modeled by an offset sinusoidal, beta(l) = -2.85 + 0.17 sin(2l - 90 degrees). Finally, we study synchrotron emission in the BICEP2 field, in an attempt to understand whether the claimed detection of large-scale B-mode polarization could be explained in terms of synchrotron contamination. Adopting a spectral index of beta = -3.12, typical for high Galactic latitudes, we find that the most likely bias corresponds to about 2% of the reported signal (r = 0.003). The flattest index allowed by the data in this region is beta = -2.5, and under the assumption of a straight power-law frequency spectrum, we find that synchrotron emission can account for at most 20% of the reported BICEP2 signal.