S. Yatawatta et al., "Initial deep LOFAR observations of epoch
of reionization windows - I. The north celestial pole", Astronomy
& Astrophysics, V550, no. A136, pp1-17, February 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 microJy/PSF in
the NCP window. Close to the NCP, the noise level increases to about
180 microJy/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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