Multi - frequency polarimetry of the Galactic radio background around 350 MHz : I . A region in Auriga around l = 161 ◦ , b = 16 ◦

With the Westerbork Synthesis Radio Telescope (WSRT), multi-frequency polarimetric images were taken of the diffuse radio synchrotron background in a ∼ 5◦×7◦ region centered on (l, b) = (161◦, 16◦) 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′×5.0′ cosec δ. The polarized intensity P and polarization angle φ 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 r.m.s. 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 <∼ RM <∼ 10 rad m −2 with a non-zero average RM0 ≈ −3.4 rad m . 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 per degree. The gradient and average RM are consistent with a regular magnetic field of ∼ 1 μG 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 |RM | <∼ 13 rad m , with an estimated intrinsic source contribution of ∼ 3.6 rad m. 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 Bran/Breg ≈ 0.7± 0.5. The polarization horizon, beyond which most diffuse polarized emission is depolarized, is estimated to be at a distance of about 600 pc.