Assessing the impact of two independent direction-dependent calibration algorithms on the LOFAR 21-cm signal power spectrum

TL;DR

This study compares two direction-dependent calibration algorithms, DDECAL and SAGECAL, for LOFAR 21-cm signal detection, finding that while they perform similarly after foreground removal, including a beam model improves calibration in the primary beam region.

Contribution

The paper introduces DDECAL, a new direction-dependent calibration algorithm, and compares its performance with SAGECAL in the context of LOFAR 21-cm experiments, highlighting the importance of beam modeling.

Findings

DDECAL performs better at source subtraction in the primary beam region.

Including a beam model significantly improves calibration performance.

Both algorithms yield comparable 21-cm power spectra after foreground removal.

Abstract

Detecting the 21-cm signal from the Epoch of Reionisation (EoR) is challenging due to the strong astrophysical foregrounds, ionospheric effects, radio frequency interference and instrumental effects. Understanding and calibrating these effects are crucial for the detection. In this work, we introduce a newly developed direction-dependent (DD) calibration algorithm DDECAL and compare its performance with an existing algorithm, SAGECAL, in the context of the LOFAR-EoR 21-cm power spectrum experiment. In our data set, the North Celestial Pole (NCP) and its flanking fields were observed simultaneously. We analyse the NCP and one of its flanking fields. The NCP field is calibrated by the standard pipeline, using SAGECAL with an extensive sky model and 122 directions, and the flanking field is calibrated by DDECAL and SAGECAL with a simpler sky model and 22 directions. Additionally, two strategies are used for subtracting Cassiopeia A and Cygnus A. The results show that DDECAL performs better at subtracting sources in the primary beam region due to the application of a beam model, while SAGECAL performs better at subtracting Cassiopeia A and Cygnus A. This indicates that including a beam model during DD calibration significantly improves the performance. The benefit is obvious in the primary beam region. We also compare the 21-cm power spectra on two different fields. The results show that the flanking field produces better upper limits compared to the NCP in this particular observation. Despite the minor differences between DDECAL and SAGECAL due to the beam application, we find that the two algorithms yield comparable 21-cm power spectra on the LOFAR-EoR data after foreground removal. Hence, the current LOFAR-EoR 21-cm power spectrum limits are not likely to depend on the DD calibration method.