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

¹ Institut d'Astrophysique Spatiale, Centre Universitaire d'Orsay, Bât. 121, 91405 Orsay Cedex, France e-mail: jonathan.aumont@ias.u-psud.fr
² Centre d'Étude Spatiale des Rayonnements, Université de Toulouse, CNRS, 9 Av. du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
³ European Space Astronomy Center, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁴ IRAM – Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d'Hères, France
⁵ IRAM – Institut de Radioastronomie Millimétrique, Avenida Divina Pastora 7, Núcleo Central, 18012 Granada, Spain
⁶ LERMA/LRA, CNRS UMR 8112, École Normale Supérieure and Observatoire de Paris, 24 rue Lhomond, 75231 Paris Cedex 05, France
⁷ Laboratoire de Physique Subatomique et de Cosmologie, Université Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique de Grenoble, 53 avenue des Martyrs, 38026 Grenoble, France
⁸ Dipartimento di Fisica, Università di Roma “La Sapienza”, Rome, Italy
⁹ APC, Université Denis Diderot-Paris 7, CNRS/IN2P3, CEA, Observatoire de Paris, 10 rue A. Domon & L. Duquet, Paris, France
¹⁰ European Space Agency, Astrophysics Division, Keplerlaan 1, 2201AZ Noordwijk, The Netherlands
¹¹ LAL – Laboratoire de l'Accélérateur Linéaire, Université Paris-Sud 11, CNRS/IN2P3, Bât. 200, 91898 Orsay Cedex, France

Received: 9 December 2009
Accepted: 16 February 2010

Abstract

Context. 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.

Aims. Our goal was to measure the Crab nebula polarization characteristics at 90 GHz with unprecedented precision.

Methods. The observations were carried out with the IRAM 30 m telescope employing the correlation polarimeter XPOL and using two orthogonally polarized receivers.

Results. 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 α_Sky = 152.1±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 α_Sky = 149.9±0.2° and a mean polarization fraction of Π = 8.8±0.2%.