The effect of systematics on polarized spectral indices

TL;DR

This study investigates how systematics affect the estimation of polarized spectral indices in bright compact objects using WMAP data, revealing that spectral index estimates are sensitive to polarization angles and may be unreliable at certain scales.

Contribution

The paper demonstrates that spectral index estimation from WMAP polarization data is significantly affected by systematics, especially detector polarization angles, highlighting potential calibration concerns.

Findings

Spectral index varies with polarization angle in WMAP data.

Spectral index estimates are unreliable at 1-degree scales.

Simple models can reproduce observed spectral index variations.

Abstract

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. A simple elliptical Gaussian toy model with parameters matching those observed in Tau A reproduces the observed signal, and shows that the spectral index is in particular sensitive to the detector polarization angle. Based on these findings, we first conclude that estimation of spectral indices with the WMAP K-band polarization data at 1 degree scales is not robust. Second, we note that these issues may be of concern for ground-based and sub-orbital experiments that use the WMAP polarization measurements of Tau A for calibration of gain and polarization angles.