Initial deep LOFAR observations of epoch of reionization windows

I. The north celestial pole

¹ University of Groningen, Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
² ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands
e-mail: yatawatta@astron.nl
³ Max-Planck Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, 85748 Garching bei München, Germany
⁴ Center for Astrophysics and Space Astronomy, University of Colorado, 389 UCB, Boulder, Colorado 80309-0389, USA
⁵ Department of Astronomy & Oskar Klein Centre, AlbaNova, Stockholm University, 10691 Stockholm, Sweden
⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ Observatoire de Paris, 92195 Meudon, France
⁸ University of Amsterdam, Astronomical Institute Anton Pannekoek, PO Box 94249, 1090 GE Amsterdam, The Netherlands
⁹ Max-Planck Institute for Astrophysics, PO Box 20 24, 53010 Bonn, Germany
¹⁰ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
¹¹ Sydney Institute for Astronomy, School of Physics A28, University of Sydney, NSW 2006, Australia
¹² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹³ Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
¹⁴ Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany
¹⁵ Astrophysical Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
¹⁶ University of Southampton, University Road, Southampton SO17 1BJ, UK
¹⁷ University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁸ Astronomisches Institut der Ruhr-Universität Bochum, Universitaetsstrasse 150, 44780 Bochum, Germany
¹⁹ Thüringer Landessternwarte, Tautenburg Observatory, Sternwarte 5, 07778 Tautenburg, Germany
²⁰ Radboud University Nijmegen, Faculty of NWI, PO Box 9010, 6500 GL Nijmegen, The Netherlands
²¹ Centre national de la recherche scientifique, 3 rue Michel-Ange, 75794 Paris Cedex 16, France
²² Jodrell Bank Center for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
²³ University of Oxford, Wellington Square, Oxford OX1 2JD, UK
²⁴ Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, 9 Av. Charles André, 69561 Saint Genis Laval Cedex, France
²⁵ Rhodes University, RATT, Dep. Physics and Electronics, PO Box 94, 6140 Grahamstown, South Africa
²⁶ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
²⁷ UCL, Department of Physics & Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
²⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²⁹ Onsala Space Observatory, Dept. of Earth and Space Sciences, Chalmers University of Technology, 43992 Onsala, Sweden
³⁰ Laboratoire Lagrange, UMR7293, Universitè de Nice Sophia-Antipolis, CNRS, Observatoire de la Cóte d’Azur, 06300 Nice, France
³¹ Astro Space Center of the Lebedev Physical Institute, Profsoyuznaya str. 84/32, 117997 Moscow, Russia
³² Center for Information Technology (CIT), University of Groningen, The Netherlands
³³ Mount Stromlo Observatory, RSAA, Cotter Road, Weston Creek, ACT 2611, Australia

Received: 7 December 2012
Accepted: 8 January 2013

Abstract

Aims. The aim of the LOFAR epoch of reionization (EoR) project is to detect the spectral fluctuations of the redshifted HI 21 cm signal. This signal is weaker by several orders of magnitude than the astrophysical foreground signals and hence, in order to achieve this, very long integrations, accurate calibration for stations and ionosphere and reliable foreground removal are essential.

Methods. One of the prospective observing windows for the LOFAR EoR project will be centered at the north celestial pole (NCP). We present results from observations of the NCP window using the LOFAR highband antenna (HBA) array in the frequency range 115 MHz to 163 MHz. The data were obtained in April 2011 during the commissioning phase of LOFAR. We used baselines up to about 30 km. The data was processed using a dedicated processing pipeline which is an enhanced version of the standard LOFAR processing pipeline.

Results. With about 3 nights, of 6 h each, effective integration we have achieved a noise level of about 100 μJy/PSF in the NCP window. Close to the NCP, the noise level increases to about 180 μJy/PSF, mainly due to additional contamination from unsubtracted nearby sources. We estimate that in our best night, we have reached a noise level only a factor of 1.4 above the thermal limit set by the noise from our Galaxy and the receivers. Our continuum images are several times deeper than have been achieved previously using the WSRT and GMRT arrays. We derive an analytical explanation for the excess noise that we believe to be mainly due to sources at large angular separation from the NCP. We present some details of the data processing challenges and how we solved them.

Conclusions. Although many LOFAR stations were, at the time of the observations, in a still poorly calibrated state we have seen no artefacts in our images which would prevent us from producing deeper images in much longer integrations on the NCP window which are about to commence. The limitations present in our current results are mainly due to sidelobe noise from the large number of distant sources, as well as errors related to station beam variations and rapid ionospheric phase fluctuations acting on bright sources. We are confident that we can improve our results with refined processing.