Wide-field LOFAR-LBA power-spectra analyses: Impact of calibration, polarization leakage and ionosphere

TL;DR

This study analyzes LOFAR-LBA data to understand calibration, polarization leakage, and ionospheric effects impacting 21 cm cosmology, revealing significant instrumental polarization leakage and extremely small ionospheric diffractive scales.

Contribution

It provides a detailed characterization of calibration and ionospheric effects at low frequencies, informing mitigation strategies for future Epoch of Reionization experiments.

Findings

Polarization leakage causes pitchfork structure in power spectrum

Extremely small ionospheric diffractive scale (~430 m) measured

Instrumental polarization leakage (~30%) linked to bright off-field source

Abstract

Contamination due to foregrounds (Galactic and Extra-galactic), calibration errors and ionospheric effects pose major challenges in 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 centered on 3C196 using LOFAR Low Band (56-70 MHz) 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 window). We show that this structure largely arises due to strong instrumental polarization leakage ($\sim30\%$) towards {Cas\,A} ($\sim21$ 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 ($r_{\text{diff}} \approx 430$ m at 60 MHz) towards {Cas\,A} resembling pure Kolmogorov turbulence compared to $r_{\text{diff}} \sim3 - 20$ 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 Cosmic Dawn observations (e.g. with SKA-low and HERA) in the same frequency window.