Initial deep LOFAR observations of epoch of reionization windows. I. The north celestial pole

S Yatawatta 1 A. G. de Bruyn 2 P Labropoulos 2 S Kazemi S Zaroubi 2 G Bernardi 3 B Ciardi 4 S Daiboo 2 G Harker 5 G Mellema 6 J Schaye 7 R. Thomas H Vedantham 2 E Chapman A Alexov 1 J Anderson 4 F Batejat 4 M Bentum 1 P Best 8 A Bonafede 9 J Bregman 1 F Breitling M Brüggen 9 J Conway F de Gasperin 10 E de Geus S Duscha 1 H Falcke 11 C Ferrari 12 W Frieswijk 1 Jean-Mathias Grießmeier 13 M Hoeft 14 M Iacobelli 7 E Juette 15 A Karastergiou 16 M Kramer 17 M Kuniyoshi 17 G Kuper 1 J van Leeuwen 18 P Maat 1 G Mann 19 M Mevius 1 H Munk 20 R Nijboer 1 E Orru 1 H Paas 21 M Pandey-Pommier 22 R Pizzo 1 W Reich 17 J Sluman 1 O Smirnov B Stappers M Steinmetz 19 Michel Tagger 13 Y Tang 1 C Tasse 23 S ter Veen 24 R Vermeulen 1 M Wise 1 O Wucknitz 25 P Zarka 26
Abstract : Aims. The aim of the LOFAR Epoch of Reionization (EoR) project is to detect the spectral fluctuations of the redshifted HI 21cm 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 hours 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.
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S Yatawatta, A. G. de Bruyn, P Labropoulos, S Kazemi, S Zaroubi, et al.. Initial deep LOFAR observations of epoch of reionization windows. I. The north celestial pole. Astronomy and Astrophysics - A&A, EDP Sciences, 2013, 550, pp.A136. ⟨10.1051/0004-6361/201220874⟩. ⟨insu-01291446⟩

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