Observations of carbon radio recombination lines with the NenuFAR telescope

I. Cassiopeia A and Cygnus A

¹ Laboratoire de Physique de l’École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 75005 Paris, France
² LUX, Observatoire de Paris, Université PSL, Sorbonne Université, 75014 Paris, France
³ Institute of Radio Astronomy NAS of Ukraine, 4, Mystetstv St., Kharkiv 61002, Ukraine
⁴ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, 75014 Paris, France
⁵ Green Bank Observatory, Green Bank, WV 24944, USA
⁶ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁸ Université Paris Cité and Université Paris Saclay, CEA, CNRS, AIM, 91190 Gif-sur-Yvette, France
⁹ Instituto de Física Fundamental (CSIC), Calle Serrano 121-123, 28006 Madrid, Spain
¹⁰ LPC2E, OSUC, Univ. Orléans, CNRS, CNES, Observatoire de Paris, Université PSL, 45071 Orléans, France
¹¹ ORN, Observatoire de Paris, Université PSL, Université d’Orléans, CNRS, 18330 Nançay, France
¹² Department of Physics and Astronomy, The University of Western Ontario, 1151 Richmond Street, London, Ontario N6A 3K7, Canada
¹³ Research School of Astronomy & Astrophysics, Australian National University, Canberra, ACT 2610, Australia
¹⁴ Laboratoire Univers et Particules de Montpellier (LUPM) Université Montpellier, CNRS/IN2P3, CC72, place Eugène Bataillon, F-34095 Montpellier Cedex 5, France
¹⁵ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁶ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁷ Leiden Observatory, Leiden University, Leiden, The Netherlands

^★ Corresponding author: lucie.cros@phys.ens.fr

Received: 8 April 2025
Accepted: 21 July 2025

Abstract

Context. Carbon radio recombination lines (CRRLs) at decametre wavelengths trace the diffuse phase of the interstellar medium (ISM) of the Galaxy. Observations of these lines allow for physical parameters of this phase to be measured.

Aims. We observed CRRLs with the recently commissioned New Extension in Nançay 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). We then measured the density, n_e, and temperature, T_e, of the electrons in line-of-sight clouds.

Methods. We used NenuFAR’s beam-forming mode and integrated several tens of hours on each source. The nominal spectral resolution was 95.4 Hz. We developed a reduction pipeline primarily aimed at removing the radio frequency interference (RFI) contamination and correcting the baselines. We then performed a first fitting of the spectral lines observed in absorption associated with the line-of-sight clouds.

Results. 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 Cα lines between the principal quantum numbers n = 426 and n = 826. Cyg A is also a bright source, however, the Cα lines were observed to be fainter. We stacked the signal by groups of a few tens of lines to improve the quality of our fitting process. For both sources, we reached a significantly higher signal-to-noise ratio (S/N) and spectral resolution than the most recent detections by the LOw Frequency ARray (LOFAR). The variation of the spectral line widths with the electronic quantum number provides constraints on the physical properties of the clouds: T_e, n_e, and the temperature, T₀, of the radiation field, the mean turbulent velocity, ν_t, and the typical size of the cloud.

Conclusions. Our final constraints differ from those inferred from LOFAR results, with ∼50% lower T_e, ∼35% lower n_e, and from 10 to 80% higher ν_t, on average. The NenuFAR observations sample a larger space volume than LOFAR’s towards the same sources due to the differences in instrumental beam sizes. These 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.