Spatial variations in the spectral index of polarized synchrotron emission in the 9 yr WMAP sky maps
- Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, NO-0315 Oslo (Norway)
We estimate the spectral index, β, of polarized synchrotron emission as observed in the 9 yr Wilkinson Microwave Anisotropy Probe sky maps using two methods, linear regression ({sup T}-T plot{sup )} 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° in steps of 5°. Averaging over polarization angles, we derive a mean spectral index of β{sup 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 β{sup plane} = –2.98 ± 0.01 and β{sup 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, β(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 β = –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 β = –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.
- OSTI ID:
- 22365509
- Journal Information:
- Astrophysical Journal, Vol. 790, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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