We study the angular power spectra of polarized Galactic synchrotron in the range 10 < ∼ l ≤ 800, at several frequencies between 0.4 and 2.7 GHz and at several Galactic latitudes up to near the North Galactic Pole. Electric-and magnetic-parity polarization spectra are found to have slopes around α E,B = 1.4 − 1.5 in the Parkes and Effelsberg Galactic-Plane surveys, but strong local fluctuations of α E,B are found at |b| ≃ 10 • from the 1.4 GHz Effelsberg survey. The C P Il spectrum, which is insensitive to the polarization direction, is somewhat steeper, being α P I = 1.6 − 1.8 for the same surveys. The low-resolution multifrequency survey of Brouw and Spoelstra (1976) shows some flattening of the spectra below 1 GHz, more intense for C E,Bl than for C P Il . In no case we find evidence for really steep spectra. The extrapolation to the cosmological window shows that at 90 GHz the detection of E-mode harmonics in the cosmic background radiation should not be disturbed by synchrotron, even around l ≃ 10 for a reionization optical depth τ ri > ∼ 0.05.

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We present the first observation of the diffuse polarized synchrotron radiation of a patch (∼ 3 • × 3 • ) in the BOOMERanG field, one of the areas with the lowest CMB foreground emission. The work has been carried out with the Australia Telescope Compact Array at 1.4 GHz with 3.4 arcmin resolution and sensitivity of ∼ 0.18 mJy beam −1 . The mean polarized signal has been found to be P rms = (Q 2 rms + U 2 rms ) = 11.6 ± 0.6 mK, nearly one order of magnitude below than in the Galactic Plane. Extrapolations to frequencies of interest for cosmological investigations suggest that polarized synchrotron foreground noise should allow the detection of the CMB Polarization E-mode already at 32 GHz and make us confident that, at 90 GHz, it is accessible with no relevant foreground contamination. Last but not least, even the B-mode detection for T /S > 0.01 is not ruled out in such a low emission patch.

We analyse the polarized emission at 1.4 GHz in a 3 • × 3 • area at high Galactic latitude (b ∼ −40 • ). The region, centred in (α = 5 h , δ = −49 • ), was observed with the Australia Telescope Compact Array (ATCA) radio-interferometer, whose 3-30 arcmin angular sensitivity range allows the study of scales appropriate for cosmic microwave background polarization (CMBP) investigations. The angular behaviour of the diffuse emission is analysed through the E-and B-mode angular power spectra. These follow a power law C X ∝ β X with slopes β E = −1.97 ± 0.08 and β B = −1.98 ± 0.07. The emission is found to be approximately a factor 25 fainter than in Galactic plane regions. The comparison of the power spectra with other surveys indicates that this area is intermediate between strong and negligible Faraday rotation effects. A similar conclusion can be reached by analysing both the frequency and Galactic latitude behaviours of the diffuse Galactic emission of the 408-1411 MHz Leiden survey data. We present an analysis of the Faraday rotation effects on the polarized power spectra and find that the observed power spectra can be enhanced by a transfer of power from large to small angular scales. The extrapolation of the spectra to 32 and 90 GHz of the cosmic microwave background (CMB) window suggests that Galactic synchrotron emission leaves the CMBP E-mode uncontaminated at 32 GHz. The level of the contamination at 90 GHz is expected to be more than 4 orders of magnitude below the CMBP spectrum. Extrapolating to the relevant angular scales, this region also appears adequate for investigation of the CMBP B-modes for models with tensor-to-scalar fluctuation power ratio T/S > 0.01. We also identify polarized point sources in the field, providing a nine object list, which is complete down to the polarized flux limit of S p lim = 2 mJy.

Abstract. We analyze the instrumental polarization generated by the antenna system (optics and feed horn) due to the unpolarized sky emission. Our equations show that it is given by the convolution of the unpolarized emission map T b (θ, φ) with a sort of instrumental polarization beam Π defined by the co-and cross-polar patterns of the antenna. This result is general, it can be applied to all antenna systems and is valid for all schemes to detect polarization, like correlation and differential polarimeters. The axisymmetric case is attractive: it generates an E-mode-like Π pattern, the contamination does not depend on the scanning strategy and the instrumental polarization map does not have B-mode contamination, making axisymmetric systems suitable to detect the faint B-mode signal of the Cosmic Microwave Background Polarization. The E-mode of the contamination only affects the FWHM scales leaving the larger ones significantly cleaner. Our analysis is also applied to the SPOrt experiment where we find that the contamination of the E-mode is negligible in the -range of interest for CMBP large angular scale investigations (multipole < 10).