Systematic biases in low frequency radio interferometric data due to calibration: the LOFAR EoR case

TL;DR

This paper investigates systematic biases in LOFAR EoR data caused by calibration with incomplete sky models, revealing how calibration can suppress diffuse signals and introduce excess noise, impacting 21 cm cosmology measurements.

Contribution

It identifies calibration with incomplete sky models as a key source of biases in low frequency radio interferometry, and discusses potential mitigation strategies.

Findings

Calibration with incomplete models causes diffuse foreground suppression.

Calibration introduces excess noise beyond thermal levels.

Using only long baselines or multi-frequency calibration can reduce biases.

Abstract

The redshifted 21 cm line of neutral hydrogen is a promising probe of the Epoch of Reionization (EoR). However, its detection requires a thorough understanding and control of the systematic errors. We study two systematic biases observed in the LOFAR EoR residual data after calibration and subtraction of bright discrete foreground sources. The first effect is a suppression in the diffuse foregrounds, which could potentially mean a suppression of the 21 cm signal. The second effect is an excess of noise beyond the thermal noise. The excess noise shows fluctuations on small frequency scales, and hence it can not be easily removed by foreground removal or avoidance methods. Our analysis suggests that sidelobes of residual sources due to the chromatic point spread function and ionospheric scintillation can not be the dominant causes of the excess noise. Rather, both the suppression of diffuse foregrounds and the excess noise can occur due to calibration with an incomplete sky model containing predominantly bright discrete sources. We show that calibrating only on bright sources can cause suppression of other signals and introduce an excess noise in the data. The levels of the suppression and excess noise depend on the relative flux of sources which are not included in the model with respect to the flux of modeled sources. We discuss possible solutions such as using only long baselines to calibrate the interferometric gain solutions as well as simultaneous multi-frequency calibration along with their benefits and shortcomings.