Initial LOFAR observations of Epoch of Reionization windows: II. Diffuse polarized emission in the ELAIS-N1 field

摘要

This study aims to characterise the polarized foreground emission in theELAIS-N1 field and to address its possible implications for the extraction ofthe cosmological 21-cm signal from the Low-Frequency Array - Epoch ofReionization (LOFAR-EoR) data. We use the high band antennas of LOFAR to imagethis region and RM-synthesis to unravel structures of polarized emission athigh Galactic latitudes. The brightness temperature of the detected Galacticemission is on average 4 K in polarized intensity and covers the range from -10to +13rad m^-2 in Faraday depth. The total polarized intensity and polarizationangle show a wide range of morphological features. We have also used theWesterbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the sameregion. The LOFAR and WSRT images show a similar complex morphology, atcomparable brightness levels, but their spatial correlation is very low. Thefractional polarization at 150 MHz, expressed as a percentage of the totalintensity, amounts to 1.5%. There is no indication of diffuse emission in totalintensity in the interferometric data, in line with results at higherfrequencies. The wide frequency range, good angular resolution and goodsensitivity make LOFAR an exquisite instrument for studying Galactic polarizedemission at a resolution of 1-2 rad m^-2 in Faraday depth. The differentpolarised patterns observed at 150 MHz and 350 MHz are consistent withdifferent source distributions along the line of sight wring in a variety ofFaraday thin regions of emission. The presence of polarised foregrounds is aserious complication for Epoch of Reionization experiments. To avoid theleakage of polarized emission into total intensity, which can depend onfrequency, we need to calibrate the instrumental polarization across the fieldof view to a small fraction of 1%.