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.

Methods: data analysis

Instrumentation: interferometers

Dark ages, reionization, first stars

Techniques: interferometric

Author

S. Yatawatta

Netherlands Institute for Radio Astronomy (ASTRON)

University of Groningen

A. G. de Bruyn

University of Groningen

Netherlands Institute for Radio Astronomy (ASTRON)

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

Harvard-Smithsonian Center for Astrophysics

University of Groningen

B. Ciardi

Max Planck Institute for Astrophysics

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 Institute for Astrophysics

I. M. Avruch

Netherlands Institute for Space Research (SRON)

University of Groningen

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 Institute for Astrophysics

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

Leiden University

Netherlands Institute for Radio Astronomy (ASTRON)

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 Manchester

University of Southampton

J. W. T. Hessels

Netherlands Institute for Radio Astronomy (ASTRON)

University of Amsterdam

M. Hoeft

Thüringer Landessternwarte Tautenburg

M. Iacobelli

Leiden University

E. Juette

Ruhr-Universität Bochum

A. Karastergiou

University of Oxford

V. I. Kondratiev

Netherlands Institute for Radio Astronomy (ASTRON)

Astro Space Center of Lebedev Physical Institute

M. Kramer

University of Manchester

Max Planck Institute for Astrophysics

M. Kuniyoshi

Max Planck Institute for Astrophysics

G. Kuper

Netherlands Institute for Radio Astronomy (ASTRON)

J. van Leeuwen

Netherlands Institute for Radio Astronomy (ASTRON)

University of Amsterdam

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

Radboud University

Netherlands Institute for Radio Astronomy (ASTRON)

H. Paas

University of Groningen

M. Pandey-Pommier

Leiden University

Lyon Observatory

R. Pizzo

Netherlands Institute for Radio Astronomy (ASTRON)

A. G. Polatidis

Netherlands Institute for Radio Astronomy (ASTRON)

W. Reich

Max Planck Institute for Astrophysics

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

Netherlands Institute for Radio Astronomy (ASTRON)

Leiden University

M. Wise

Netherlands Institute for Radio Astronomy (ASTRON)

O. Wucknitz

Argelander-Institut für Astronomie

Max Planck Institute for Astrophysics

P. Zarka

Observatoire de Paris-Meudon

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 550 A136

Subject Categories

Astronomy, Astrophysics and Cosmology

DOI

10.1051/0004-6361/201220874

More information

Latest update

7/8/2021 6