A sensitive upper limit to the circular polarization of the Crab nebula at λ3 mm

¹ Institut de Radioastronomie Millimétrique 300 rue de la Piscine, 38406 Saint Martin d’Hères France
e-mail: hwiese@mpifr.de
² Institut d’Astrophysique Spatiale, Centre Universitaire d’Orsay, Bât. 121, 91405 Orsay Cedex, France
³ Astroparticule et Cosmologie, Université Paris-Diderot, Bâtiment Condorcet, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France

Received: 13 December 2010
Accepted: 29 December 2010

Abstract

A new observation of the distribution of the circular polarization over the Crab Nebula supernova remnant yields an upper limit of  <0.2% at a radio frequency of 89.2 GHz. This limit is set by the uncertainty in correcting for the instrumental polarization. The raw data were dominated by the conversion of the strong linear polarization to circular in the crosspolarized sidelobes of the 30 m telescope. They were modeled as due to a differential phase gradient between the orthogonally linearly polarized far-field radiation patterns of the two receivers. As the source is tracked these rotate with respect to the radio source distribution on the sky since the telescope has an alt-azimuth mount and a Nasmyth focus. This allows the model to be fit to the raw data and a correction can be made. Our limit of  <0.2% is to be compared with  <0.03% derived at 610 MHz (Wilson & Weiler 1997, ApJ, 475, 661) and  <6% measured at 23 GHz (Wright & Forster 1980, ApJ, 239, 873). These limits are consistent with the polarization expected from an optically thin synchrotron source with the known physical properties of the Crab Nebula. This non-detection does not allow an estimate to be made of the relative contribution to the radio emission from electrons and positrons.