The polarized thermal emission from Galactic dust is the main foreground present in measurements of the polarization of the cosmic microwave background (CMB) at frequencies above 100GHz. We exploit the Planck HFI polarization data from 100 to 353GHz
to measure the dust angular power spectra $C_ell^{EE,BB}$ over the range $40<ell<600$ well away from the Galactic plane. These will bring new insights into interstellar dust physics and a precise determination of the level of contamination for CMB polarization experiments. We show that statistical properties of the emission can be characterized over large fractions of the sky using $C_ell$. For the dust, they are well described by power laws in $ell$ with exponents $alpha^{EE,BB}=-2.42pm0.02$. The amplitudes of the polarization $C_ell$ vary with the average brightness in a way similar to the intensity ones. The dust polarization frequency dependence is consistent with modified blackbody emission with $beta_d=1.59$ and $T_d=19.6$K. We find a systematic ratio between the amplitudes of the Galactic $B$- and $E$-modes of 0.5. We show that even in the faintest dust-emitting regions there are no clean windows where primordial CMB $B$-mode polarization could be measured without subtraction of dust emission. Finally, we investigate the level of dust polarization in the BICEP2 experiment field. Extrapolation of the Planck 353GHz data to 150GHz gives a dust power $ell(ell+1)C_ell^{BB}/(2pi)$ of $1.32times10^{-2}mu$K$_{CMB}^2$ over the $40<ell<120$ range; the statistical uncertainty is $pm0.29$ and there is an additional uncertainty (+0.28,-0.24) from the extrapolation, both in the same units. This is the same magnitude as reported by BICEP2 over this $ell$ range, which highlights the need for assessment of the polarized dust signal even in the cleanest windows of the sky.
CMB experiments aiming at a precise measurement of the CMB polarization, such as the Planck satellite, need a strong polarized absolute calibrator on the sky to accurately set the detectors polarization angle and the cross-polarization leakage. As th
e most intense polarized source in the microwave sky at angular scales of few arcminutes, the Crab nebula will be used for this purpose. Our goal was to measure the Crab nebula polarization characteristics at 90 GHz with unprecedented precision. The observations were carried out with the IRAM 30m telescope employing the correlation polarimeter XPOL and using two orthogonally polarized receivers. We processed the Stokes I, Q, and U maps from our observations in order to compute the polarization angle and linear polarization fraction. The first is almost constant in the region of maximum emission in polarization with a mean value of alpha_Sky=152.1+/-0.3 deg in equatorial coordinates, and the second is found to reach a maximum of Pi=30% for the most polarized pixels. We find that a CMB experiment having a 5 arcmin circular beam will see a mean polarization angle of alpha_Sky=149.9+/-0.2 deg and a mean polarization fraction of Pi=8.8+/-0.2%.
