Spectral modelling of Cygnus A between 110 and 250 MHz. Impact on the LOFAR 21-cm signal power spectrum

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 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.