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CMB experiments aiming at a precise measurement of the CMB polarization, suchas the Planck satellite, need a strong polarized absolute calibrator on the skyto accurately set the detectors polarization angle and the cross-polarizationleakage. As the most intense polarized source in the microwave sky at angularscales of few arcminutes, the Crab nebula will be used for this purpose. Ourgoal was to measure the Crab nebula polarization characteristics at 90 GHz withunprecedented precision. The observations were carried out with the IRAM 30mtelescope employing the correlation polarimeter XPOL and using two orthogonallypolarized receivers. We processed the Stokes I, Q, and U maps from ourobservations in order to compute the polarization angle and linear polarizationfraction. The first is almost constant in the region of maximum emission inpolarization with a mean value of alpha_Sky=152.1+/-0.3 deg in equatorialcoordinates, and the second is found to reach a maximum of Pi=30% for the mostpolarized pixels. We find that a CMB experiment having a 5 arcmin circular beamwill see a mean polarization angle of alpha_Sky=149.9+/-0.2 deg and a meanpolarization fraction of Pi=8.8+/-0.2%.

Measurement of the Crab nebula polarization at 90 GHz as a calibrator for CMB experiments