Polarized radio filaments outside the Galactic plane

We used data from the WMAP satellite at 23, 33 and 41 GHz to study the diffuse polarized emission over the entire sky. The emission originates mostly from filamentary structures with well-ordered magnetic fields. Some of these structures have been known for decades in radio continuum maps. Their origin is not clear and there are many filaments that are visible for the first time. We have identified and studied 11 filaments. The polarization fraction of some of them can be as high as 40 per cent, which is a signature of a well-ordered magnetic field. The polarization spectral indices, averaged over 18 regions in the sky is β = -3.06 ± 0.02, consistent with synchrotron radiation. There are significant variations in β over the sky (Δβ ≈ 0.2). We explore the link between the large-scale filaments and the local interstellar medium, using the model of an expanding shell in the solar vicinity. We compared observed polarization angles with the predictions from the model and found good agreement. This strongly suggests that many large-scale filaments and loops are nearby structures. This is important in the context of the Galactic magnetic field as these structures are normally included in global models, neglecting the fact that they might be local. We also studied the level of contamination added by the diffuse filaments to the CMB (cosmic microwave background) polarization power spectra. We conclude that, even though these filaments present low radio brightness, a careful removal will be necessary for future all-sky CMB polarization analysis.