Linear polarization structures in LOFAR observations of the interstellar medium in the 3C 196 field ^⋆

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
e-mail: vjelic@astro.rug.nl
² ASTRON – The Netherlands Institute for Radio Astronomy, PO Box 2, 7990 AA Dwingeloo, The Netherlands
³ Ruđer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia
⁴ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁷ SKA SA, 3rd Floor, The Park, Park Road, 7405 Pinelands, South Africa
⁸ Max-Planck Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, 85748 Garching bei München, Germany
⁹ Astronomy Centre, Department of Physics & Astronomy, Pevensey II Building, University of Sussex, Brighton BN1 9QH, UK
¹⁰ Department of Astronomy and Oskar Klein Centre, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
¹¹ ASTRON & IBM Center for Exascale technology, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands

Received: 30 May 2015
Accepted: 2 September 2015

Abstract

Aims. This study aims to characterize linear polarization structures in LOFAR observations of the interstellar medium (ISM) in the 3C 196 field, one of the primary fields of the LOFAR-Epoch of Reionization key science project.

Methods. We have used the high band antennas (HBA) of LOFAR to image this region and rotation measure (RM) synthesis to unravel the distribution of polarized structures in Faraday depth.

Results. The brightness temperature of the detected Galactic emission is 5−15 K in polarized intensity and covers the range from –3 to +8 rad m^-2 in Faraday depth. The most interesting morphological feature is a strikingly straight filament at a Faraday depth of +0.5 rad m^-2 running from north to south, right through the centre of the field and parallel to the Galactic plane. There is also an interesting system of linear depolarization canals conspicuous in an image showing the peaks of Faraday spectra. We used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. For the first time, we see some common morphology in the RM cubes made at 150 and 350 MHz. There is no indication of diffuse emission in total intensity in the interferometric data, in line with results at higher frequencies and previous LOFAR observations. Based on our results, we determined physical parameters of the ISM and proposed a simple model that may explain the observed distribution of the intervening magneto-ionic medium.

Conclusions. The mean line-of-sight magnetic field component, B_∥, is determined to be 0.3 ± 0.1 μG and its spatial variation across the 3C 196 field is 0.1 μG. The filamentary structure is probably an ionized filament in the ISM, located somewhere within the Local Bubble. This filamentary structure shows an excess in thermal electron density (n_eB_∥> 6.2 cm^-3μG) compared to its surroundings.