Initial deep LOFAR observations of Epoch of Reionization windows: I. The
  North Celestial Pole
  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. 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. With about 3
nights, of 6 hours 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.
