Transient RFI environment of LOFAR-LBA at 72-75 MHz: Impact on ultra-widefield AARTFAAC Cosmic Explorer observations of the redshifted 21-cm signal

Abstract

Measurement of the redshifted 21-cm signal of neutral hydrogen from the Cosmic Dawn (CD) and Epoch of Reionisation (EoR) promises to unveil a wealth of information about the astrophysical processes during the first billion years of evolution of the universe. The AARTFAAC Cosmic Explorer (ACE) utilises the AARTFAAC wide-field imager of LOFAR to measure the power spectrum of the intensity fluctuations of the redshifted 21-cm signal from the CD at z~18. The RFI from various sources contaminates the observed data and it is crucial to exclude the RFI-affected data in the analysis for reliable detection. In this work, we investigate the impact of non-ground-based transient RFI using cross-power spectra and cross-coherence metrics to assess the correlation of RFI over time and investigate the level of impact of transient RFI on the ACE 21-cm power spectrum estimation. We detected moving sky-based transient RFI sources that cross the field of view within a few minutes and appear to be mainly from aeroplane communication beacons at the location of the LOFAR core in the 72-75 MHz band, by inspecting filtered images. This transient RFI is mostly uncorrelated over time and is only expected to dominate over the thermal noise for an extremely deep integration time of 3000 hours or more with a hypothetical instrument that is sky temperature dominated at 75 MHz. We find no visible correlation over different k-modes in Fourier space in the presence of noise for realistic thermal noise scenarios. We conclude that the sky-based transient RFI from aeroplanes, satellites and meteorites at present does not pose a significant concern for the ACE analyses at the current level of sensitivity and after integrating over the available 500 hours of observed data. However, it is crucial to mitigate or filter such transient RFI for more sensitive experiments aiming for significantly deeper integration.

13 pages, 9 figures, and 3 tables; accepted for publication in Astronomy and Astrophysics (A&A)

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• Broadband Polarized Radio Emission Detected from Starlink Satellites Below 100 MHz with NenuFAR
• The impact of lossy data compression on the power spectrum of the high redshift 21-cm signal with LOFAR
• Measuring the locations and properties of VHF sources emitted from an aircraft flying through high clouds
• The impact of lunar topography on the 21-cm power spectrum for grid-based arrays : Insights for the Dark-ages EXplorer (DEX)
• Mitigating incoherent excess variance in high-redshift 21 cm observations with multi-output cross-Gaussian process regression
• Beyond the Power Spectrum: A New Framework for Non-Stationary Fields with Applications to Light-Cone Effects in Line Intensity Mapping