Improved upper limits on the 21 cm signal power spectrum of neutral hydrogen at z ≈ 9.1 from LOFAR

Mertens, Florent G.; Mevius, M.; Koopmans, Leon V. E.; Offringa, A. R.; Mellema, Garrelt; Zaroubi, Saleem; Brentjens, M. A.; Gan, H.; Gehlot, Bharat Kumar; Pandey, V. N.; Sardarabadi, A. M.; Vedantham, Harish K.; Yatawatta, Sarod; Asad, Khan M. B.; Ciardi, B.; Chapman, Emma; Gazagnes, S.; Ghara, R.; Ghosh, Abhik; Giri, Sambit Kumar; Iliev, Ilian T.; Jelić, Vibor; Kooistra, Robin; Mondal, Rajesh; Schaye, Joop; Silva, Marta Bruno
Journal article; PublishedVersion; Peer reviewed
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Year
2020
Permanent link
http://urn.nb.no/URN:NBN:no-82429

CRIStin
1827431

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Original version
Monthly notices of the Royal Astronomical Society. 2020, 493 (2), 1662-1685, DOI: https://doi.org/10.1093/mnras/staa327
Abstract
A new upper limit on the 21 cm signal power spectrum at a redshift of z ≈ 9.1 is presented, based on 141 h of data obtained with the Low-Frequency Array (LOFAR). The analysis includes significant improvements in spectrally smooth gain-calibration, Gaussian Process Regression (GPR) foreground mitigation and optimally weighted power spectrum inference. Previously seen ‘excess power’ due to spectral structure in the gain solutions has markedly reduced but some excess power still remains with a spectral correlation distinct from thermal noise. This excess has a spectral coherence scale of 0.25–0.45 MHz and is partially correlated between nights, especially in the foreground wedge region. The correlation is stronger between nights covering similar local sidereal times. A best 2-σ upper limit of Δ221<(73)2mK2