© CC
The image shows the high-resolution LOFAR image of Cygnus A (left panel) alongside the spectral fitting of its two brightest hotspots, A and D (right panels). The spectral energy distribution of these hotspots deviates from a simple power law, instead exhibiting a turnover at around 160 MHz that agrees with the current literature. This feature is now incorporated in the new model by using the forced-spectrum method (see https://www.astron.nl/dailyimage/main.php?date=20230818) during a multi-scale and multi-frequency deconvolution, correcting past assumptions of a fixed spectral index across the source. The total intensity of the new Cygnus A model has been rescaled to agree with the literature values between 10 and 300 MHz, making it suitable for calibration pipelines such as LINC, where it should soon be available.
We also tested the impact of this improved model on LOFAR 21-cm power spectrum measurements on the North Celestial Pole field. Significant improvements are observed along the Cygnus A direction, highlighting the importance of spectrally accurate models for bright sources. This study highlights the importance of precise sky models for future 21-cm experiments, particularly as next-generation instruments like SKA move toward detecting the cosmological 21-cm signal.
The new Cygnus A model is available for download here https://zenodo.org/records/14863290
Link to the paper: Spectral modelling of Cygnus A between 110 and 250 MHz. Impact on the LOFAR 21-cm signal power spectrum https://arxiv.org/abs/2502.18459