Measurement of the Crab nebula polarization at 90 GHz as a calibrator for CMB experiments
Résumé
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 in order to accurately set the detectors polarization angle and the cross-polarization leakage. The Crab Nebula, as the most intense polarized source in the microwave sky at angular scales of few arcminutes, will be used for this purpose. Our goal was to measure the Crab nebula polarization characteristics at 90 GHz with an unprecedented precision. The observations were carried out with the IRAM 30m telescope employing the correlation polarimeter XPOL and using two orthogonally polarized receivers. We have 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+/-0.2 deg 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%.