A numerical study of 21-cm signal suppression and noise increase in direction-dependent calibration of LOFAR data

M. Mevius; F. Mertens; L. V. E. Koopmans; A. R. Offringa; S. Yatawatta; M. A. Brentjens; E. Chapman; B. Ciardi; H. Gan; B. K. Gehlot; R. Ghara; A. Ghosh; S. K. Giri; I. T. Iliev; G. Mellema; V. N. Pandey; S. Zaroubi

doi:10.1093/mnras/stab3233

A numerical study of 21-cm signal suppression and noise increase in direction-dependent calibration of LOFAR data

M. Mevius^*, F. Mertens, L. V. E. Koopmans, A. R. Offringa, S. Yatawatta, M. A. Brentjens, E. Chapman, B. Ciardi, H. Gan, B. K. Gehlot, R. Ghara, A. Ghosh, S. K. Giri, I. T. Iliev, G. Mellema, V. N. Pandey, S. Zaroubi

^*Corresponding author for this work

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Abstract

We investigate systematic effects in direction-dependent gain calibration in the context of the Low-Frequency Array (LOFAR) 21-cm Epoch of Reionization (EoR) experiment. The LOFAR EoR Key Science Project aims to detect the 21-cm signal of neutral hydrogen on interferometric baselines of 50-250 λ. We show that suppression of faint signals can effectively be avoided by calibrating these short baselines using only the longer baselines. However, this approach causes an excess variance on the short baselines due to small gain errors induced by overfitting during calibration. We apply a regularized expectation-maximization algorithm with consensus optimization (SAGECAL-CO) to real data with simulated signals to show that overfitting can be largely mitigated by penalising spectrally non-smooth gain solutions during calibration. This reduces the excess power with about a factor of 4 in the simulations. Our results agree with earlier theoretical analysis of this bias-variance trade off and support the gain-calibration approach to the LOFAR 21-cm signal data.

Original language	English
Pages (from-to)	3693-3702
Number of pages	10
Journal	Monthly Notices of the Royal Astronomical Society
Volume	509
Issue number	3
DOIs	https://doi.org/10.1093/mnras/stab3233
Publication status	Published - 1-Jan-2022

Keywords

methods: numerical
methods: observational
techniques: interferometric
dark ages
reionization
first stars
Astrophysics - Cosmology and Nongalactic Astrophysics
Astrophysics - Instrumentation and Methods for Astrophysics

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A numerical study of 21-cm signal suppression and noise increase in direction-dependent calibration of LOFAR dataFinal publisher's version, 4 MBLicence: Unspecified

https://ui.adsabs.harvard.edu/abs/2022MNRAS.509.3693M

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Mevius, M., Mertens, F., Koopmans, L. V. E., Offringa, A. R., Yatawatta, S., Brentjens, M. A., Chapman, E., Ciardi, B., Gan, H., Gehlot, B. K., Ghara, R., Ghosh, A., Giri, S. K., Iliev, I. T., Mellema, G., Pandey, V. N., & Zaroubi, S. (2022). A numerical study of 21-cm signal suppression and noise increase in direction-dependent calibration of LOFAR data. Monthly Notices of the Royal Astronomical Society, 509(3), 3693-3702. https://doi.org/10.1093/mnras/stab3233

@article{093da132c37b4edd94dbf25dfd85bc04,

title = "A numerical study of 21-cm signal suppression and noise increase in direction-dependent calibration of LOFAR data",

abstract = "We investigate systematic effects in direction-dependent gain calibration in the context of the Low-Frequency Array (LOFAR) 21-cm Epoch of Reionization (EoR) experiment. The LOFAR EoR Key Science Project aims to detect the 21-cm signal of neutral hydrogen on interferometric baselines of 50-250 λ. We show that suppression of faint signals can effectively be avoided by calibrating these short baselines using only the longer baselines. However, this approach causes an excess variance on the short baselines due to small gain errors induced by overfitting during calibration. We apply a regularized expectation-maximization algorithm with consensus optimization (SAGECAL-CO) to real data with simulated signals to show that overfitting can be largely mitigated by penalising spectrally non-smooth gain solutions during calibration. This reduces the excess power with about a factor of 4 in the simulations. Our results agree with earlier theoretical analysis of this bias-variance trade off and support the gain-calibration approach to the LOFAR 21-cm signal data.",

keywords = "methods: numerical, methods: observational, techniques: interferometric, dark ages, reionization, first stars, Astrophysics - Cosmology and Nongalactic Astrophysics, Astrophysics - Instrumentation and Methods for Astrophysics",

author = "M. Mevius and F. Mertens and Koopmans, {L. V. E.} and Offringa, {A. R.} and S. Yatawatta and Brentjens, {M. A.} and E. Chapman and B. Ciardi and H. Gan and Gehlot, {B. K.} and R. Ghara and A. Ghosh and Giri, {S. K.} and Iliev, {I. T.} and G. Mellema and Pandey, {V. N.} and S. Zaroubi",

year = "2022",

month = jan,

day = "1",

doi = "10.1093/mnras/stab3233",

language = "English",

volume = "509",

pages = "3693--3702",

journal = "Monthly Notices of the Royal Astronomical Society",

issn = "0035-8711",

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Mevius, M, Mertens, F , Koopmans, LVE, Offringa, AR, Yatawatta, S, Brentjens, MA, Chapman, E, Ciardi, B, Gan, H, Gehlot, BK, Ghara, R, Ghosh, A, Giri, SK, Iliev, IT, Mellema, G, Pandey, VN & Zaroubi, S 2022, 'A numerical study of 21-cm signal suppression and noise increase in direction-dependent calibration of LOFAR data', Monthly Notices of the Royal Astronomical Society, vol. 509, no. 3, pp. 3693-3702. https://doi.org/10.1093/mnras/stab3233

TY - JOUR

T1 - A numerical study of 21-cm signal suppression and noise increase in direction-dependent calibration of LOFAR data

AU - Mevius, M.

AU - Mertens, F.

AU - Koopmans, L. V. E.

AU - Offringa, A. R.

AU - Yatawatta, S.

AU - Brentjens, M. A.

AU - Chapman, E.

AU - Ciardi, B.

AU - Gan, H.

AU - Gehlot, B. K.

AU - Ghara, R.

AU - Ghosh, A.

AU - Giri, S. K.

AU - Iliev, I. T.

AU - Mellema, G.

AU - Pandey, V. N.

AU - Zaroubi, S.

PY - 2022/1/1

Y1 - 2022/1/1

N2 - We investigate systematic effects in direction-dependent gain calibration in the context of the Low-Frequency Array (LOFAR) 21-cm Epoch of Reionization (EoR) experiment. The LOFAR EoR Key Science Project aims to detect the 21-cm signal of neutral hydrogen on interferometric baselines of 50-250 λ. We show that suppression of faint signals can effectively be avoided by calibrating these short baselines using only the longer baselines. However, this approach causes an excess variance on the short baselines due to small gain errors induced by overfitting during calibration. We apply a regularized expectation-maximization algorithm with consensus optimization (SAGECAL-CO) to real data with simulated signals to show that overfitting can be largely mitigated by penalising spectrally non-smooth gain solutions during calibration. This reduces the excess power with about a factor of 4 in the simulations. Our results agree with earlier theoretical analysis of this bias-variance trade off and support the gain-calibration approach to the LOFAR 21-cm signal data.

AB - We investigate systematic effects in direction-dependent gain calibration in the context of the Low-Frequency Array (LOFAR) 21-cm Epoch of Reionization (EoR) experiment. The LOFAR EoR Key Science Project aims to detect the 21-cm signal of neutral hydrogen on interferometric baselines of 50-250 λ. We show that suppression of faint signals can effectively be avoided by calibrating these short baselines using only the longer baselines. However, this approach causes an excess variance on the short baselines due to small gain errors induced by overfitting during calibration. We apply a regularized expectation-maximization algorithm with consensus optimization (SAGECAL-CO) to real data with simulated signals to show that overfitting can be largely mitigated by penalising spectrally non-smooth gain solutions during calibration. This reduces the excess power with about a factor of 4 in the simulations. Our results agree with earlier theoretical analysis of this bias-variance trade off and support the gain-calibration approach to the LOFAR 21-cm signal data.

KW - methods: numerical

KW - methods: observational

KW - techniques: interferometric

KW - dark ages

KW - reionization

KW - first stars

KW - Astrophysics - Cosmology and Nongalactic Astrophysics

KW - Astrophysics - Instrumentation and Methods for Astrophysics

U2 - 10.1093/mnras/stab3233

DO - 10.1093/mnras/stab3233

M3 - Article

SN - 0035-8711

VL - 509

SP - 3693

EP - 3702

JO - Monthly Notices of the Royal Astronomical Society

JF - Monthly Notices of the Royal Astronomical Society

IS - 3

ER -

A numerical study of 21-cm signal suppression and noise increase in direction-dependent calibration of LOFAR data

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Keywords

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