1201.6348 (Ingunn Kathrine Wehus et al.)

The effect of asymmetric beams on polarized spectral indices    [PDF]

Ingunn Kathrine Wehus, Unni Fuskeland, Hans Kristian Eriksen

We study four particularly bright polarized compact objects (Tau A, Virgo A, 3C273 and Fornax A) in the 7-year WMAP sky maps, with the goal of understanding potential systematics involved in estimation of foreground spectral indices. We estimate the spectral index, the polarization angle, the polarization fraction and apparent size and shape of these objects when smoothed to a nominal resolution of 1 degree FWHM. Second, we compute the spectral index as a function of polarization orientation, alpha. Because these objects are approximately point sources with constant polarization angle, this function should be constant in the absence of systematics. However, computing it for the K- and Ka-band WMAP data we find strong index variations for all four sources. For Tau A, we find a spectral index beta=-2.59+-0.03 for alpha=30 degrees, and beta=-2.03+-0.01 for alpha=50 degrees. On the other hand, the spectral index between Ka and Q band is found to be stable. The most likely cause of this effect is beam asymmetries, which effectively results in a leakage of the foreground amplitude into the spectral index. A simple toy model of an asymmetric beam produces qualitatively very similar results. Based on these findings, we therefore conclude that estimation of spectral indices with the WMAP K-band polarization data at 1 degree scales is not robust. Second, we argue that a model with a common spectral index for the Stokes' Q and U parameters is likely to produce better results than a model with independent indices. The methods used in this paper are likely to be useful to identify systematics in other experiments as well.

View original: http://arxiv.org/abs/1201.6348