Initial LOFAR observations of epoch of reionization windows

II. Diffuse polarized emission in the ELAIS-N1 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 Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁴ SKA SA, 3rd Floor, The Park, Park Road, 7405 Pinelands, South Africa
⁵ Max-Planck Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, 85748 Garching bei München, Germany
⁶ Center for Astrophysics and Space Astronomy, 389 University of Colorado, Boulder CO 80309, USA
⁷ Department of Astronomy and Oskar Klein Centre, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
⁸ RSAA, Australian National University, Mt Stromlo Observatory, via Cotter Road, Weston, ACT 2611, Australia
⁹ ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), Australia
¹⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, USA
¹¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹² SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
¹³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁵ University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁶ School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
¹⁷ Onsala Space Observatory, Dept. of Earth and Space Sciences, Chalmers University of Technology, 43992 Onsala, Sweden
¹⁸ Astronomisches Institut der Ruhr-Universität Bochum, Universitaetsstrasse 150, 44780 Bochum, Germany
¹⁹ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
²⁰ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
²¹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
²² Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
²³ Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France
²⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²⁵ LPC2E – Université d’Orléans/CNRS, Orléans, France
²⁶ Station de Radioastronomie de Nançay, Observatoire de Paris – CNRS/INSU, USR 704 – Univ. Orléans, OSUC, route de Souesmes, 18330 Nançay, France
²⁷ Jodrell Bank Center for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
²⁸ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
²⁹ Astro Space Center of the Lebedev Physical Institute, Profsoyuznaya str. 84/32, 117997 Moscow, Russia
³⁰ Sodankylä Geophysical Observatory, University of Oulu, Tähteläntie 62, 99600 Sodankylä, Finland
³¹ STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK
³² Center for Information Technology (CIT), University of Groningen, The Netherlands
³³ Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, 9 Av. Charles André, 69561 Saint Genis Laval Cedex, France
³⁴ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
³⁵ Department of Physics and Electronics, Rhodes University, PO Box 94, 6140 Grahamstown, South Africa
³⁶ LESIA, UMR CNRS 8109, Observatoire de Paris, 92195 Meudon, France
³⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA
³⁸ Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 15 April 2014
Accepted: 7 July 2014

Abstract

Aims. This study aims to characterise the polarized foreground emission in the ELAIS-N1 field and to address its possible implications for extracting of the cosmological 21 cm signal from the LOw-Frequency ARray – Epoch of Reionization (LOFAR-EoR) data.

Methods. We used the high band antennas of LOFAR to image this region and RM-synthesis to unravel structures of polarized emission at high Galactic latitudes.

Results. The brightness temperature of the detected Galactic emission is on average ~4 K in polarized intensity and covers the range from –10 to + 13 rad m^-2 in Faraday depth. The total polarized intensity and polarization angle show a wide range of morphological features. We have also used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. The LOFAR and WSRT images show a similar complex morphology at comparable brightness levels, but their spatial correlation is very low. The fractional polarization at 150 MHz, expressed as a percentage of the total intensity, amounts to ≈1.5%. There is no indication of diffuse emission in total intensity in the interferometric data, in line with results at higher frequencies

Conclusions. The wide frequency range, high angular resolution, and high sensitivity make LOFAR an exquisite instrument for studying Galactic polarized emission at a resolution of ~1–2 rad m^-2 in Faraday depth. The different polarized patterns observed at 150 MHz and 350 MHz are consistent with different source distributions along the line of sight wring in a variety of Faraday thin regions of emission. The presence of polarized foregrounds is a serious complication for epoch of reionization experiments. To avoid the leakage of polarized emission into total intensity, which can depend on frequency, we need to calibrate the instrumental polarization across the field of view to a small fraction of 1%.