Spectral modelling of Cygnus A between 110 and 250 MHz

Impact on the LOFAR 21-cm signal power spectrum

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
² INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
³ ASTRON, PO Box 2 7990 AA Dwingeloo, The Netherlands
⁴ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, F-75014 Paris, France
⁵ Max-Planck Institute for Astrophysics, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany
⁶ Department of Physical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur, WB 741246, India
⁷ Laboratoire de Physique de l’ENS, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 75005 Paris, France
⁸ Astronomy Centre, Department of Physics & Astronomy, Pevensey III Building, University of Sussex, Falmer, Brighton BN1 9QH, United Kingdom
⁹ South African Radio Astronomy Observatory (SARAO), PO Box 443 Krugersdorp 1740, South Africa
¹⁰ Department of Physics, University of Pretoria, Lynnwood Road, Hatfield, Pretoria 0083, South Africa
¹¹ Department of Natural Sciences, The Open University of Israel, 1 University Road, PO Box 808 Ra’anana 4353701, Israel

^⋆ Corresponding author; emilio.ceccotti@inaf.it

Received: 21 November 2024
Accepted: 24 February 2025

Abstract

Studying the redshifted 21-cm signal from the neutral hydrogen during the Epoch of Reionisation and Cosmic Dawn is fundamental for understanding the physics of the early universe. One of the challenges that 21-cm experiments face is the contamination by bright foreground sources, such as Cygnus A, for which accurate spatial and spectral models are needed to minimise the residual contamination after their removal. In this work, we develop a new, high-resolution model of Cygnus A using Low Frequency Array (LOFAR) observations in the 110–250 MHz range, improving upon previous models by incorporating physical spectral information through the forced-spectrum method during multi-frequency deconvolution. This approach addresses the limitations of earlier models by providing a more accurate representation of the complex structure and spectral behaviour of Cygnus A, including the spectral turnover in its brightest hotspots. The impact of this new model on the LOFAR 21-cm signal power spectrum is assessed by comparing it with both simulated and observed North Celestial Pole datasets. Significant improvements are observed in the cylindrical power spectrum along the Cygnus A direction, highlighting the importance of having spectrally accurate models of the brightest foreground sources. However, this improvement is washed out in the spherical power spectrum, where we measure differences of a few hundred mK at k < 0.63 h cMpc⁻¹, but not statistically significant. The results suggest that other systematic effects must be mitigated before a substantial impact on 21-cm power spectrum can be achieved.