First upper limits on the 21-cm signal power spectrum of neutral hydrogen at <i>z</i> = 9.16 from the LOFAR 3C 196 field

ABSTRACT The redshifted 21-cm signal of neutral hydrogen from the epoch of reionization (EoR) can potentially be detected using low-frequency radio instruments such as the Low-Frequency Array (LOFAR). So far, LOFAR upper limits on the 21-cm signal power spectrum have been published using a single target field: the North Celestial Pole (NCP). In this work, we analyse and provide upper limits for the 3C 196 field, observed by LOFAR, with a strong ${\approx }80$ Jy source in the centre. This field offers advantages such as higher sensitivity due to zenith-crossing observations and reduced geostationary radio-frequency interference, but also poses challenges due to the presence of the bright central source. After constructing a wide-field sky model, we process a single 6-h night of 3C 196 observations using direction-independent and direction-dependent calibration, followed by a residual foreground subtraction with a machine learning Gaussian process regression (ML-GPR). A bias correction is necessary to account for signal suppression in the GPR step. Still, even after this correction, the upper limits are a factor of 2 lower than previous single-night NCP results, with a lowest $2\sigma$ upper limit of $(146.61\, \text{mK})^2$ at redshift $z = 9.16$ and wavenumber $k=0.078\, h\, \text{cMpc}^{-1}$ (with $\text{d}k/k\approx 0.3$). The results also reveal an excess power, different in behaviour from that observed in the NCP field, suggesting a potential residual foreground origin. In future work, the use of multiple nights of 3C 196 observations combined with improvements to sky modelling and ML-GPR to avoid the need for bias correction should provide tighter constraints per unit observing time than the NCP.

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Publication Details

Journal
Monthly Notices of the Royal Astronomical Society
Published
2025-09-26
DOI
https://doi.org/10.1093/mnras/staf1629
Citations
5
Primary Topic
Radio Astronomy Observations and Technology
Type
article
Field-Weighted Citation Impact
5.77

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article

First upper limits on the 21-cm signal power spectrum of neutral hydrogen at z = 9.16 from the LOFAR 3C 196 field

Sambit K. Giri, S. Munshi, Emma Chapman, B. Ciardi et al.
5 citations
Monthly Notices of the Royal Astronomical Society
Radio Astronomy Observations and Technology
5.77
article

First upper limits on the 21-cm signal power spectrum of neutral hydrogen at z = 9.16 from the LOFAR 3C 196 field

Sambit K. Giri, S. Munshi, Emma Chapman, B. Ciardi, Christian Hofer, Garrelt Mellema, Ian Hothi, S. A. Brackenhoff, A. R. Offringa, Raghunath Ghara, Florent Mertens, Anshuman Acharya, E Ceccotti, J. Kariuki Chege, L. V. E. Koopmans, M. Mevius, Saleem Zaroubi, V. N. Pandey, S. Ghosh
article en
5 citations

Abstract

ABSTRACT The redshifted 21-cm signal of neutral hydrogen from the epoch of reionization (EoR) can potentially be detected using low-frequency radio instruments such as the Low-Frequency Array (LOFAR). So far, LOFAR upper limits on the 21-cm signal power spectrum have been published using a single target field: the North Celestial Pole (NCP). In this work, we analyse and provide upper limits for the 3C 196 field, observed by LOFAR, with a strong ${\approx }80$ Jy source in the centre. This field offers advantages such as higher sensitivity due to zenith-crossing observations and reduced geostationary radio-frequency interference, but also poses challenges due to the presence of the bright central source. After constructing a wide-field sky model, we process a single 6-h night of 3C 196 observations using direction-independent and direction-dependent calibration, followed by a residual foreground subtraction with a machine learning Gaussian process regression (ML-GPR). A bias correction is necessary to account for signal suppression in the GPR step. Still, even after this correction, the upper limits are a factor of 2 lower than previous single-night NCP results, with a lowest $2\sigma$ upper limit of $(146.61\, \text{mK})^2$ at redshift $z = 9.16$ and wavenumber $k=0.078\, h\, \text{cMpc}^{-1}$ (with $\text{d}k/k\approx 0.3$). The results also reveal an excess power, different in behaviour from that observed in the NCP field, suggesting a potential residual foreground origin. In future work, the use of multiple nights of 3C 196 observations combined with improvements to sky modelling and ML-GPR to avoid the need for bias correction should provide tighter constraints per unit observing time than the NCP.

Monthly Notices of the Royal Astronomical SocietyVol. 544(1)
Indian Institute of Science Education and Research Kolkata (IN), Centre National de la Recherche Scientifique (FR), University of Nottingham (GB), Stockholm University (SE), Open University of Israel (IL), University of Groningen (NL), Université Paris Cité (FR), Université Paris Sciences et Lettres (FR), École Normale Supérieure - PSL (FR), Sorbonne Université (FR), Sorbonne Paris Cité (FR), Max Planck Institute for Astrophysics (DE), Istituto di Radioastronomia di Bologna (IT), AlbaNova (SE), Astronomical Institute of the Slovak Academy of Sciences (SK), KTH Royal Institute of Technology (SE), Netherlands Institute for Radio Astronomy (NL)
European Commission, Department of Science and Technology, Ministry of Science and Technology, India, Science Foundation Ireland, Rijksuniversiteit Groningen, Bundesministerium für Bildung und Forschung, Nederlandse Organisatie voor Wetenschappelijk Onderzoek, Max-Planck-Gesellschaft, Vetenskapsrådet, Centre National de la Recherche Scientifique, Istituto Nazionale di Astrofisica, Université d'Orléans, Ministerium für Innovation, Wissenschaft und Forschung des Landes Nordrhein-Westfalen, Observatoire de Paris, Université de Recherche Paris Sciences et Lettres, Science and Technology Facilities Council, Science and Engineering Research Board, Army Research Office
Openalex Percentile: Top 4%
Radio Astronomy Observations and Technology
5.77
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