Tentative detection of circularly polarized bursty radio emissions from the HD 189733 exoplanetary system using NenuFAR beamformed observations

Technical Summary: Tentative Detection of Circularly Polarized Bursty Radio Emissions from the HD 189733 Exoplanetary System

Problem and Context
The detection of exoplanetary magnetic fields remains a primary challenge in planetary science, with auroral radio emission via the cyclotron maser instability (CMI) considered the most promising detection method. While decades of ground-based observations have yielded unambiguous non-detections, recent tentative detections in the $\tau$ Boötis and HD 189733 systems have renewed interest. Specifically, Zhang et al. (2025, hereafter Z25) reported a 6$\sigma$ detection of a highly circularly polarized radio burst at 50 MHz from the HD 189733 system using NenuFAR imaging observations. However, the origin of this emission remains ambiguous, with four potential scenarios proposed: sub-Alfvénic star-planet interaction (SPI), wind-magnetosphere planetary auroral emission, stellar CMI, or stellar plasma emission from flares. The Z25 study utilized imaging data but lacked an independent verification using beamformed data, which offers higher sensitivity to attenuated bursts.

Methodology
This study analyzes NenuFAR beamformed observations of the HD 189733 system taken simultaneously with the imaging data that yielded the Z25 detection (September 28, 2023, 19:31–22:47 UT). The dataset includes an "ON-beam" targeting the system and three "OFF-beams" targeting source-free regions to characterize terrestrial ionospheric fluctuations, radio frequency interference (RFI), and instrumental systematics.

The analysis employed the BOREALIS (BeamfOrmed Radio Emission AnaLysIS) pipeline, previously validated on LOFAR Jupiter data and NenuFAR pulsar data. The workflow involved:

1. Pre-processing: Generation of Level 2 (L2) data computing Stokes parameters (I, Q, U, V), applying RFI mitigation, and correcting for instrumental gain variations.
2. Filtering: High-pass filtering of the Stokes V (circularly polarized) dynamic spectra to remove large-scale systematics (e.g., baseline changes and grating lobe artifacts).
3. Statistical Analysis: Calculation of burst observables ($Q2$, $Q3$, $Q4$) comparing the ON-beam against OFF-beams. The $Q2$ observable provides a time-series comparison, while $Q3$ and $Q4$ offer statistical measures of burst significance against a reference curve of 10,000 draws of Gaussian noise.
4. Search Parameters: The search covered multiple frequency ranges (21–58 MHz, with specific focus on 27–40 MHz and the Z25 47–52 MHz range) and time resolutions ($\Delta\tau$ of 0.1, 1, and 8 seconds).

Key Results
The analysis yielded a tentative detection of circularly polarized bursty emission distinct from the Z25 imaging result in both timing and frequency:

• Detection Significance: A strong signal was identified in the 27–40 MHz range between 19:40 and 19:56 UT (approximately 1 hour prior to the Z25 imaging burst). The detection reached a significance of 7.4$\sigma$ in the $Q4f$ statistic and 10$\sigma$ in the $Q3f$ statistic when compared to Gaussian noise.
• Signal Characteristics: The emission was left-hand polarized (V- analysis) with a duration of approximately 16 minutes, composed of individual bursts lasting ~1 second. The estimated flux density is ~3.9 Jy, corresponding to a brightness temperature ($T_B$) of $4–6 \times 10^{16}$ K.
• Comparison with Z25: Unlike the Z25 imaging detection (right-hand polarized, 47.6–52.1 MHz, 96s duration, 1.5 Jy), this beamformed detection occurred at a lower frequency, earlier in time, and exhibited different polarization and duration characteristics.
• Noise Analysis: The authors identified excess correlated noise in the data, visible as a diagonal cloud of points in diagnostic scatter plots (Figure 2A–B) and non-Gaussian behavior in OFF-beam residuals. While the $Q4$ statistics are designed to ignore correlated diagonal data, the presence of this noise prevents a conclusive claim of astrophysical origin.

Interpretation and Constraints
Assuming an astrophysical origin, the authors evaluate the four proposed scenarios:

1. Stellar Plasma Emission: Ruled out as the primary mechanism because the observed brightness temperature ($10^{16}$ K) vastly exceeds the theoretical saturation limit for plasma emission ($\sim 10^{12}$ K).
2. Sub-Alfvénic SPI and Stellar CMI: The observed frequency range implies a plasma-to-cyclotron frequency ratio ($f_{pe}/f_{ce}$) of 2.1–2.3, which is too high to suppress the CMI mechanism unless the stellar magnetic field is exceptionally strong (150–200 G).
3. Wind-Magnetosphere Interaction: This scenario is deemed the most consistent with the observations. The ~1 second burst duration resembles Jupiter's S-bursts, and the timing aligns with radiative transfer models suggesting planetary radio emission escapes primarily near primary transit. This scenario implies a planetary magnetic field constraint of $\sim$12–18 G.

Significance and Claims
The paper claims a tentative detection of circularly polarized bursty radio emission from the HD 189733 system. The authors emphasize that while the signal is statistically significant relative to Gaussian noise, the presence of excess correlated noise (potentially low-level RFI or systematics unique to NenuFAR) precludes a definitive confirmation.

The study highlights the complementary power of simultaneous beamformed and imaging observations. The beamformed data detected a burst at a different time and frequency than the imaging data, suggesting that exoplanetary radio emissions may be highly variable and that single-epoch observations may miss significant activity. The authors conclude that further low-frequency radio observations are required to confirm the astrophysical nature of the signal, search for periodicity, and definitively distinguish between planetary and stellar origins.