3c196 Flanking Field

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Clik here to view.© LOFAR EoR Collaboration

Four Hundred Million years since the start of time, the Dark Ages of the Universe ended with the birth of the first generation stars. These stars ionized the surrounding neutral Hydrogen which was the most abundant cosmic gas present then. This cosmic era is referred to as the Epoch of Reionization (EoR). The High-band Antennas of LOFAR probe the EoR using high-redshift 21-cm Hydrogen spin-flip transition. For the EoR science LOFAR has several target fields one of them being the 3c196 field.

In December 2016, the sky around 3c196 was observed with two beams. A flanking field was observed and analysed to see if it was a better prospect for EoR power-spectrum analysis given that the bright 3c196 source is attenuated by the edge of the beam. The foregrounds in this field were removed with a novel technique that entails manually modelling the brightest sources and automatically extracting sources using wsclean's multiscale option. The frequency stacked skyimage in Figure(a) shows the result of this source model subtraction on all the subbands between 114-133 MHz, along with its power spectrum. The power in the long and short baselines outside the foreground wedge is seen to be low. But this can be reduced further. The skyimage shows artefacts due to the sidelobes and ionosphere which contributes power to the power spectrum.

With the same source model, direction-dependent calibration was carried out to remove the detrimental effects due to the ionosphere and beam modelling errors. Figure(b) is the result of source subtraction on one frequency subband after it was calibrated in 25 independent directions. The power as indicated by the power spectrum has reduced considerably. These results indicate that this chosen approach of foreground removal is quite effective and the new 3c196 flanking indeed looks promising for EoR analysis.