Spatial variations in the spectral index of polarized synchrotron emission in the 9 yr WMAP sky maps

TL;DR

This study maps the spatial variations of the polarized synchrotron spectral index across the sky using WMAP data, revealing latitude and longitude dependencies, and assesses its impact on BICEP2 B-mode polarization measurements.

Contribution

It provides detailed measurements of the polarized synchrotron spectral index variations across the sky and evaluates their implications for cosmic microwave background polarization studies.

Findings

Spectral index averages -2.99 across the sky.

Steepening of 0.14 from low to high Galactic latitudes.

Synchrotron contamination could explain up to 20% of BICEP2 B-mode signal.

Abstract

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.