There is no author summary for this article yet. Authors can add summaries to their articles on ScienceOpen to make them more accessible to a non-specialist audience.
Abstract
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 the 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%.