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

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.

28 pages, 14 figures, accepted for publication in A&A; a few equations in section 3 were corrected in this version

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• Constraints on the state of the IGM at $z\sim 8-10$ using redshifted 21-cm observations with LOFAR
• Transient RFI environment of LOFAR-LBA at 72-75 MHz: Impact on ultra-widefield AARTFAAC Cosmic Explorer observations of the redshifted 21-cm signal
• Improved upper limits on the 21-cm signal power spectrum at $z=17.0$ and $z=20.3$ from an optimal field observed with NenuFAR
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• Ionospheric contributions to the excess power in high-redshift 21-cm power-spectrum observations with LOFAR
• Spectral modelling of Cygnus A between 110 and 250 MHz. Impact on the LOFAR 21-cm signal power spectrum
• Diffuse Sources, Clustering and the Excess Anisotropy of the Radio Synchrotron Background
• First upper limits on the 21-cm signal power spectrum of neutral hydrogen at $z=9.16$ from the LOFAR 3C196 field
• Studying the Multi-frequency Angular Power Spectrum of the Cosmic Dawn 21-cm Signal
• Ionospheric effect on the synthetic Epoch of Reionization observations with the SKA1-Low
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• Mitigating incoherent excess variance in high-redshift 21 cm observations with multi-output cross-Gaussian process regression
• Lossless compression of simulated radio interferometric visibilities