Observations of Carbon Radio Recombination Lines with the NenuFAR telescope. I. Cassiopeia A and Cygnus A
Lucie Cros, Antoine Gusdorf, Philippe Salomé, Sergiy Stepkin, Philippe Zarka, Pedro Salas, Alan Loh, Pierre Lesaffre, Jonathan Freundlich, Marta Alves, François Boulanger, Andrea Bracco, Stéphane Corbel, Maryvonne Gerin, Javier Goicoechea, Isabelle Grenier, Jean-Mathias Grießmeier, Martin Houde, Oleksandr Konovalenko, Antoine Marchal, Alexandre Marcowith, Florent Mertens, Frédérique Motte, Michel Tagger, Alexander Tielens, Gilles Theureau, Peter Tokarsky, Oleg Ulyanov, Vyacheslav Zakharenko
Published: 2025/6/10
Abstract
Carbon Radio Recombination Lines (CRRLs) at decametre wavelengths trace the diffuse phase of the interstellar medium (ISM) of the Galaxy. Their observation allows to measure physical parameters of this phase. We observed CRRLs with the recently commissioned New Extension in Nan\c{c}ay Upgrading LOFAR (NenuFAR) telescope towards two of the brightest sources at low-frequency (10-85 MHz): Cassiopeia A and Cygnus A (hereafter Cas A and Cyg A respectively), to measure the density n_e and temperature T_e of electrons in line-of-sight clouds. We used NenuFAR's beamforming mode, and we integrated several tens of hours on each source. The nominal spectral resolution was 95.4 Hz. We developed a pipeline to remove radio frequency interference (RFI) contamination and correct the baselines. We then fitted the spectral lines observed in absorption, associated to line-of-sight clouds. Cas A is the brightest source in the sky at low frequencies and represents an appropriate test bench for this new telescope. On this source, we detected 398 C\alpha lines between principal quantum numbers n=426 and n=826. C\alpha lines towards Cyg A were fainter. We stacked the signal by groups of a few tens of lines to improve the quality of our fitting process. On both sources we reached significantly higher S/N and spectral resolution than the most recent detections by the LOw Frequency ARray (LOFAR). The variation of line shape with n provides constraints on the physical properties of the clouds: T_e, n_e, the temperature T_0 of the radiation field, the mean turbulent velocity v_t and the typical size of the cloud. The NenuFAR observations sample a larger space volume than LOFAR's towards the same sources due to the differences in instrumental beamsizes, and the discrepancies highlight the sensitivity of low-frequency CRRLs as probes of the diffuse ISM, paving the way towards large area surveys of CRRLs in our Galaxy.