Initial deep LOFAR observations of epoch of reionization windows I. The north celestial pole
Journal article, 2013
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 mu Jy/PSF in the NCP window. Close to the NCP, the noise level increases to about 180 mu Jy/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.
Instrumentation: interferometers
Dark ages, reionization, first stars
Techniques: interferometric
Methods: data analysis
Author
S. Yatawatta
University of Groningen
Netherlands Institute for Radio Astronomy (ASTRON)
A. G. de Bruyn
Netherlands Institute for Radio Astronomy (ASTRON)
University of Groningen
M. A. Brentjens
Netherlands Institute for Radio Astronomy (ASTRON)
P. Labropoulos
Netherlands Institute for Radio Astronomy (ASTRON)
V. N. Pandey
Netherlands Institute for Radio Astronomy (ASTRON)
S. Kazemi
University of Groningen
S. Zaroubi
University of Groningen
L. V. E. Koopmans
University of Groningen
A. R. Offringa
University of Groningen
Mount Stromlo Observatory
V. Jelic
University of Groningen
O. M. Rubi
University of Groningen
V. Veligatla
University of Groningen
S. J. Wijnholds
Netherlands Institute for Radio Astronomy (ASTRON)
W. N. Brouw
Netherlands Institute for Radio Astronomy (ASTRON)
G. Bernardi
University of Groningen
Harvard-Smithsonian Center for Astrophysics
B. Ciardi
Max Planck Society
S. Daiboo
University of Groningen
G. Harker
University of Colorado at Boulder
G. Mellema
Stockholm University
J. Schaye
Leiden University
R Thomas
University of Groningen
H. Vedantham
University of Groningen
E. Chapman
University College London (UCL)
F. B. Abdalla
University College London (UCL)
A. Alexov
Space Telescope Science Institute (STScI)
J. Anderson
Max Planck Society
I. M. Avruch
University of Groningen
Netherlands Institute for Space Research (SRON)
Fabien Batejat
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
M. E. Bell
University of Southampton
The University of Sydney
M. R. Bell
Max Planck Society
M. Bentum
Netherlands Institute for Radio Astronomy (ASTRON)
P. Best
Royal Observatory
A. Bonafede
Jacobs University Bremen
J. Bregman
Netherlands Institute for Radio Astronomy (ASTRON)
F. Breitling
Leibniz Institute for Astrophysics Potsdam
R. H. van de Brink
Netherlands Institute for Radio Astronomy (ASTRON)
J. W. Broderick
University of Southampton
M. Brüggen
Jacobs University Bremen
University of Hamburg
John Conway
Chalmers, Earth and Space Sciences, Onsala Space Observatory
F. De Gasperin
University of Hamburg
E. de Geus
Netherlands Institute for Radio Astronomy (ASTRON)
S. Duscha
Netherlands Institute for Radio Astronomy (ASTRON)
H. Falcke
Radboud University
R. A. Fallows
Netherlands Institute for Radio Astronomy (ASTRON)
C. Ferrari
Université Nice Sophia Antipolis (UNS)
W. Frieswijk
Netherlands Institute for Radio Astronomy (ASTRON)
M. A. Garrett
Netherlands Institute for Radio Astronomy (ASTRON)
Leiden University
J. M. Griessmeier
Centre national de la recherche scientifique (CNRS)
Netherlands Institute for Radio Astronomy (ASTRON)
A. W. Gunst
Netherlands Institute for Radio Astronomy (ASTRON)
T. E. Hassall
University of Southampton
University of Manchester
J. W. T. Hessels
University of Amsterdam
Netherlands Institute for Radio Astronomy (ASTRON)
M. Hoeft
Thüringer Landessternwarte Tautenburg
M. Iacobelli
Leiden University
E. Juette
Ruhr-Universität Bochum
A. Karastergiou
University of Oxford
V. I. Kondratiev
Astro Space Center of Lebedev Physical Institute
Netherlands Institute for Radio Astronomy (ASTRON)
M. Kramer
Max Planck Society
University of Manchester
M. Kuniyoshi
Max Planck Society
G. Kuper
Netherlands Institute for Radio Astronomy (ASTRON)
J. van Leeuwen
University of Amsterdam
Netherlands Institute for Radio Astronomy (ASTRON)
P. Maat
Netherlands Institute for Radio Astronomy (ASTRON)
G. Mann
Leibniz Institute for Astrophysics Potsdam
J. P. McKean
Netherlands Institute for Radio Astronomy (ASTRON)
M. Mevius
Netherlands Institute for Radio Astronomy (ASTRON)
University of Groningen
J. D. Mol
Netherlands Institute for Radio Astronomy (ASTRON)
H. Munk
Netherlands Institute for Radio Astronomy (ASTRON)
R. Nijboer
Netherlands Institute for Radio Astronomy (ASTRON)
J. E. Noordam
Netherlands Institute for Radio Astronomy (ASTRON)
M. J. Norden
Netherlands Institute for Radio Astronomy (ASTRON)
E. Orru
Netherlands Institute for Radio Astronomy (ASTRON)
Radboud University
H. Paas
University of Groningen
M. Pandey-Pommier
Lyon Observatory
Leiden University
R. Pizzo
Netherlands Institute for Radio Astronomy (ASTRON)
A. G. Polatidis
Netherlands Institute for Radio Astronomy (ASTRON)
W. Reich
Max Planck Society
H. Rottgering
Leiden University
J. Sluman
Netherlands Institute for Radio Astronomy (ASTRON)
O. Smirnov
Rhodes University
B. Stappers
University of Manchester
M. Steinmetz
Leibniz Institute for Astrophysics Potsdam
M. Tagger
Centre national de la recherche scientifique (CNRS)
Y. Tang
Netherlands Institute for Radio Astronomy (ASTRON)
C. Tasse
Observatoire de Paris-Meudon
S. ter Veen
Radboud University
R. Vermeulen
Netherlands Institute for Radio Astronomy (ASTRON)
R. J. van Weeren
Harvard-Smithsonian Center for Astrophysics
Leiden University
Netherlands Institute for Radio Astronomy (ASTRON)
M. Wise
Netherlands Institute for Radio Astronomy (ASTRON)
O. Wucknitz
Argelander-Institut für Astronomie
Max Planck Society
P. Zarka
Observatoire de Paris-Meudon
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 550 A136Subject Categories (SSIF 2011)
Astronomy, Astrophysics and Cosmology
DOI
10.1051/0004-6361/201220874