2018MNRAS.478.1484G

Contamination due to foregrounds (Galactic and extragalactic), calibration errors, and ionospheric effects poses major challenges in the detection of the cosmic 21 cm signal in various epoch of reionization (EoR) experiments. We present the results of a pilot study of a field centred on 3C196 using LOFAR (LOw Frequency ARray) low-band (56-70MHz) observations, where we quantify various wide field and calibration effects such as gain errors, polarized foregrounds, and ionospheric effects. We observe a 'pitchfork' structure in the 2D power spectrum of the polarized intensity in delay-baseline space, which leaks into the modes beyond the instrumental horizon [EoR/CD (cosmic dawn) window]. We show that this structure largely arises due to strong instrumental polarization leakage (∼30per cent) towards Cas A (∼21 kJy at 81 MHz, brightest source in northern sky), which is far away from primary field of view. We measure an extremely small ionospheric diffractive scale (r_diff ≃ 430 m at 60 MHz) towards Cas A resembling pure Kolmogorov turbulence compared to r_diff∼ 3-20 km towards zenith at 150 MHz for typical ionospheric conditions. This is one of the smallest diffractive scales ever measured at these frequencies. Our work provides insights in understanding the nature of aforementioned effects and mitigating them in future CD observations (e.g. with Square Kilometre Array-low and Hydrogen Epoch of Reionization Array) in the same frequency window.