2009/12/31 by J. Aumont, L. Conversi, C. Thum +14 · 1 citation
Physics and Astronomy · #Astrophysics #Brewster's angle #Circular polarization #Cosmic microwave background #Cosmology and Gravitation Theories #Crab Nebula #Galaxies: Formation, Evolution, Phenomena #Gamma ray #Linear polarization #Optics #Physics #Planck #Polarimeter #Polarimetry #Polarization (electrochemistry) #Radio Astronomy Observations and Technology #Scattering #Sky #Stokes parameters #Telescope #astro-ph.CO #astro-ph.GA
paper · pdf · doi:10.1051/0004-6361/200913834
Accepted for publication in A&A, 9 pages, 4 figures
arxiv created 2010/02/23 · openalex publication_date 2010/02/23 · arxiv updated 2016/06/08 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
<i>Context. <i/>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.<i>Aims. <i/>Our goal was to measure the Crab nebula polarization characteristics at 90 GHz with unprecedented precision.<i>Methods. <i/>The observations were carried out with the IRAM 30 m telescope employing the correlation polarimeter XPOL and using two orthogonally polarized receivers.<i>Results. <i/>We processed the Stokes <i>I<i/>, <i>Q<i/>, and <i>U<i/> 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 <i>α<i/><sub>Sky<sub/> = 152.1<i>±<i/>0.3° in equatorial coordinates, and the second is found to reach a maximum of Π = 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 <i>α<i/><sub>Sky<sub/> = 149.9<i>±<i/>0.2° and a mean polarization fraction of Π = 8.8<i>±<i/>0.2%.