Initial deep LOFAR observations of Epoch of Reionization windows: I. The North Celestial Pole

TL;DR

This paper reports initial LOFAR observations of the Epoch of Reionization window at the North Celestial Pole, achieving deep continuum images and analyzing noise sources to aid future 21cm signal detection.

Contribution

First LOFAR high-band observations of the NCP EoR window, demonstrating deep imaging and noise analysis during commissioning phase.

Findings

Achieved a noise level of about 100 microJy/PSF in 3 nights of data.

Deep continuum images surpass previous WSRT and GMRT results.

Identified sources of excess noise mainly from distant sources near the NCP.

Abstract

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.