2025A&A...700A.140Z

Aims. We aim to detect low-frequency radio emission from exoplanetary systems to gain insights into planetary magnetic fields, star-planet interactions, stellar activity, and exo-space weather. The HD 189733 system, hosting a well-studied hot Jupiter, is a prime target for such searches.
Methods. We conducted NenuFAR imaging observations in the 15-62 MHz range to cover the entire orbital phase of HD 189733 b. Dynamic spectra were generated for the target and other sources in the field, followed by a transient search in the time-frequency plane. The data processing pipeline incorporated direction-dependent calibration and noise characterization to improve sensitivity. We also searched for periodic signals using a Lomb-Scargle analysis.
Results. A highly circularly polarized radio burst was detected at 50 MHz, with a flux density of 1.5 Jy and a significance of 6σ at the position of HD 189733. No counterpart was found in Stokes I, likely because the emission is embedded in confusion noise and remains below the detection threshold. The estimated minimum fractional circular polarization of 38% suggests a coherent emission process. A periodicity search revealed no weaker signals linked to the planet's orbital period, the star's rotational period, or the synodic period and harmonic period between them. The burst's properties are consistent with cyclotron maser instability (CMI) emission, however, the origin remains ambiguous. A comparison with theoretical models suggests star-planet interaction or stellar activity as potential origins. Alternative explanations such as contamination from other sources along the line of sight (e.g. the companion M dwarf) or noise fluctuation are plausible.