EDP Sciences
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Issue
A&A
Volume 403, Number 3, June I 2003
Page(s) 1031 - 1044
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20030432


A&A 403, 1031-1044 (2003)
DOI: 10.1051/0004-6361:20030432

Multi-frequency polarimetry of the Galactic radio background around 350 MHz

I. A region in Auriga around l = 161°, b = 16°
M. Haverkorn1, P. Katgert2 and A. G. de Bruyn3, 4

1  Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
2  Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
    e-mail: katgert@strw.leidenuniv.nl
3  ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands
    e-mail: ger@astron.nl
4  Kapteyn Institute, PO Box 800, 9700 AV Groningen, The Netherlands

(Received 14 November 2002 / Accepted 21 March 2003)

Abstract
With the Westerbork Synthesis Radio Telescope (WSRT), multi-frequency polarimetric images were taken of the diffuse radio synchrotron background in a ~ $5\degr\times7\degr$ region centered on $(l,b) =
(161\degr,16\degr)$ in the constellation of Auriga. The observations were done simultaneously in 5 frequency bands, from 341 MHz to 375 MHz, and have a resolution of ~ $5.0\arcmin\times5.0\arcmin$ cosec  $\delta$. The polarized intensity  P and polarization angle  $\phi$ show ubiquitous structure on arcminute and degree scales, with polarized brightness temperatures up to about 13 K. On the other hand, no structure at all is observed in total intensity  I to an rms limit of 1.3 K, indicating that the structure in the polarized radiation must be due to Faraday rotation and depolarization mostly in the warm component of the nearby Galactic interstellar medium (ISM). Different depolarization processes create structure in polarized intensity  P. Beam depolarization creates "depolarization canals" of one beam wide, while depth depolarization is thought to be responsible for creating most of the structure on scales larger than a beam width. Rotation measures ( RM) can be reliably determined, and are in the range $-17 \la RM \la 10$ rad m -2 with a non-zero average $RM_0 \approx
-3.4$  rad m -2. The distribution of  RMs on the sky shows both abrupt changes on the scales of the beam and a gradient in the direction of positive Galactic longitude of ~1 rad m -2 per degree. The gradient and average  RM are consistent with a regular magnetic field of ~ $1~\mu$G which has a pitch angle of p = -14°. There are 13 extragalactic sources in the field for which  RMs could be derived, and those have $\vert RM\vert \la 13$ rad m -2, with an estimated intrinsic source contribution of ~3.6 rad m -2. The RMs of the extragalactic sources show a gradient that is about 3 times larger than the gradient in the RMs of the diffuse emission and that is approximately in Galactic latitude. This difference is ascribed to a vastly different effective length of the line of sight. The  RMs of the extragalactic sources also show a sign reversal which implies a reversal of the magnetic field across the region on scales larger than about ten degrees. The observations are interpreted in terms of a simple single-cell-size model of the warm ISM which contains gas and magnetic fields, with a polarized background. The observations are best fitted with a cell size of 10 to 20 pc and a ratio of random to regular magnetic fields $B_{\rm ran}/B_{\rm reg} \approx 0.7\pm0.5$. The polarization horizon, beyond which most diffuse polarized emission is depolarized, is estimated to be at a distance of about 600 pc.


Key words: magnetic fields -- polarization -- techniques: polarimetric -- ISM: magnetic fields -- ISM: structure -- radio continuum: ISM

Offprint request: M. Haverkorn, mhaverkorn@cfa.harvard.edu

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