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 WMAP 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
deg and 85 deg in steps of 5. 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+0.17sin(2l-90). 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.
