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

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 (∼ 30%) 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 Cosmic Dawn observations (e.g. with SKA-low and HERA) in the same frequency window.

H I 21‐cm Cosmology and the Bispectrum: Closure Diagnostics in Massively Redundant Interferometric Arrays

“…In the case of HERA-47, the field of view is about 8 ∘ , which is larger than the expected sizes for significant structure in the ionosphere of ∼1.5 ∘ (corresponding to ∼10 km at the height of the troposphere, Kassim et al, 2007). The short baselines considered here (29 m and shorter), however, mean that all the antennas will see the same ionosphere: the diffracted antenna beams at the height of the atmosphere (where the first Fresnel zone radius is around √ h 2 ∼ 200 m) largely coherently overlap, and the power fluctuations of electron density at these short length scales is small (Gehlot et al, 2017). Hence, the summed effect of the ionosphere should still cancel in a closure calculation.…”

Understanding the diversity of 21 cm cosmology analyses

“…There have been a number of different proposed methods for estimating the cosmological PSP from interferometric measurements, including the imaging PS, the variance statistic, the delay spectrum, and m-mode analysis (Zaldarriaga et al 2004;Morales & Hewitt 2004;Mellema et al 2006;Stuart et al 2008;Iliev et al 2008;Chang et al 2008;Pen et al 2009;Vedantham et al 2012;Parsons et al 2012;Masui et al 2013;Switzer et al 2013;Patil et al 2014;Shaw et al 2014;Dillon et al 2014;Paul et al 2014Paul et al , 2016Trott et al 2016;Liu et al 2016;Gehlot & Koopmans 2017). While the language used to describe the estimators is highly variable, the mathematics fall into two families depending on whether they measure the PS of the measured visibilities ('delay spectrum' approaches) or the PS of the reconstructed sky ('imaging' PS approaches).…”

“…Another variant first proposed by Vedantham et al (2012) and further developed by Paul et al (2014Paul et al ( , 2016 and Gehlot & Koopmans (2017), is to grid visibilities by their physical separation (r instead of u). This can be visualized as a skewed version of Figure 1, where the black baselines now run vertically but the fringes from a flat spectrum source run diagonally (same crossing angle, fringes run from top left to bottom right).…”