Resolved imaging confirms a radiation belt around an ultracool dwarf

Ultracool dwarfs (very low-mass stars and brown dwarfs) exhibit auroral radio emissions and quiescent radio signals that resemble planetary magnetospheric phenomena, but the physical origin of their persistent, slowly varying quiescent emission has been debated. The study targets the nearby M8.5 ultracool dwarf LSR J1835+3259 (mass ~77 Jupiter masses, radius ~1.07 Jupiter radii, distance ~5.69 pc) to test whether its quiescent radio emission originates from a large-scale, stable structure analogous to planetary radiation belts confined by a dipolar magnetic field. The research question asks whether resolved imaging can confirm a double-lobed synchrotron morphology consistent with radiation belts and thereby clarify the mechanism producing quiescent emission in ultracool dwarfs. Establishing an extrasolar radiation belt would extend planetary magnetospheric paradigms beyond the Solar System and inform how rotating magnetic dipoles generate non-thermal radio emission across the stellar and substellar regimes.

Planetary radiation belts (Earth, Jupiter, Saturn, Uranus, Neptune) comprise relativistic electrons up to tens of MeV, producing slowly varying radio emissions and influencing satellite surface chemistry. Ultracool dwarfs produce aurorae (periodic, coherent radio bursts via electron cyclotron maser instability) and quiescent radio emission that deviates from standard multiwavelength flare correlations, challenging a purely coronal flare origin. Theoretical and observational work has predicted radiation belts in both ultracool dwarfs and massive, strongly magnetized stars, suggesting rotationally powered magnetospheres can drive quiescent radio emission. Previous studies have inferred strong surface magnetic fields (kG-level) and observed correlations between quiescent radio and Balmer emission in massive stars and ultracool dwarfs, hinting at aurora-linked magnetospheric processes rather than chromospheric flaring. Prior low-frequency imaging of Jupiter’s belts and modeling of stellar magnetospheres inform expectations for morphology, polarization, and frequency-dependent spatial extent of radiation belts.

Observations: The team used the High Sensitivity Array (HSA)—combining the VLBA, phased VLA, Green Bank Telescope, and Effelsberg—to observe LSR J1835+3259 at 8.4 GHz in three five-hour epochs (15 June 2019; 20 August 2020; 28 August 2020), capturing nearly two full rotations per epoch. The target is an edge-on M8.5 ultracool dwarf (i≈90°). Pre-imaging VLA-only sessions (X band) were used to refine the target position due to high proper motion and parallax. Calibration and imaging: Standard phase-referenced VLBI reduction in AIPS was applied, using J1835+3241 as the nearby ICRF phase calibrator. Data were carefully flagged for RFI and bad visibilities; long MK–EB baselines were excluded where calibration was unreliable due to short overlap times. Proper motion and parallax during each 5 h session were computed from Gaia DR3 and corrected (AIPS CLCOR) to mitigate motion smearing. Instrumental circular polarization was characterized (∼7–10% spurious in initial HSA calibration) and reduced to ≤1% by self-calibrating on the phase calibrator and transferring amplitude solutions to the target. Phased-VLA time series (AIPS DFTPL) isolated strongly circularly polarized auroral bursts (period 2.84±0.01 h) in RCP and LCP; these intervals were removed to form quiescent-only datasets. Quiescent emission images were produced per epoch; two freely floating elliptical Gaussians were fitted to quantify lobe sizes and separations. Sensitivity to extended structure varied by epoch due to missing antennas and baseline coverage, with synthesized beams of order 1–2 mas by 0.4–0.6 mas. Target parameters for interpretation: Surface magnetic field ≥3 kG (from 8.4 GHz aurorae), distance 5.6875±0.00292 pc, mass 77.28±10.34 MJ, radius 1.07±0.05 RJ (adopted as RUCD), and edge-on inclination. Magnetic field geometry at large radii assumed dipolar (B∝r^−3). Synchrotron theory was used to infer electron Lorentz factors from the observed frequency and estimated local field at the magnetic equator. Polarization: Circular polarization in the resolved lobes was assessed; linear polarization was not calibrated in these observations and is deferred to future work.

• Spatially resolved, double-lobed, axisymmetric quiescent radio emission at 8.4 GHz was detected from LSR J1835+3259 in all three epochs, persisting over >1 year.
• Lobe separation reached up to 18.47±1.85 RUCD (Epoch 3), with centroid distances ~9 RUCD from the object and outer extents at least 12–14 RUCD; the structure may extend farther but is limited by sensitivity in some epochs.
• The auroral bursts (coherent ECMI) are centrally located between the lobes (clearly seen in Epoch 2), consistent with emission from near-surface strong fields, whereas the quiescent lobes trace large-scale magnetospheric plasma.
• No significant circular polarization was detected in the resolved lobes in any epoch; 95% confidence upper limits in the less-resolved Epoch 1 are ≥8.8% (east) and 15.5% (west), consistent with synchrotron (not gyrosynchrotron) emission.
• Interpreting the lobe centroids as near the magnetic equator with a dipolar field ≥3 kG at the surface implies local fields ~2 G and a non-relativistic cyclotron frequency ~6 MHz. Emission at 8.4 GHz thus requires very high harmonics (s≥1500), ruling out gyrosynchrotron and favoring synchrotron from highly relativistic electrons.
• Synchrotron modeling yields Lorentz factor γ≈30 for near-perpendicular pitch angles at 8.4 GHz, corresponding to electron energies ~15 MeV—comparable to Jupiter’s radiation belts.
• The 8.4 GHz lobe separation is ~70% more compact than contemporaneous 4.5 GHz measurements, consistent with higher-frequency emission tracing higher-energy electrons in more compact regions, as seen at Jupiter.
• Estimated synchrotron cooling time is ~60 days, yet the structure persists over >1 year and quiescent emission at similar frequencies has been present for over a decade, implying ongoing acceleration and/or replenishment mechanisms.
• The observed stable, equatorial, double-lobed morphology and scale are consistent with plasma confined by a global dipole, analogous to the Jovian radiation belts.

The resolved double-lobed, axisymmetric quiescent emission directly addresses the research question by confirming an extrasolar analogue of planetary radiation belts around an ultracool dwarf. The morphology, frequency-dependent compactness, lack of circular polarization, and inferred MeV electron energies are all consistent with synchrotron emission from relativistic electrons trapped in a large-scale, stable dipolar magnetosphere. This finding rules out a purely coronal flare origin for the persistent quiescent emission and supports models where rotating magnetic dipoles and magnetosphere-ionosphere coupling drive particle acceleration and confinement, akin to Jupiter’s rotationally powered system. The results align with theoretical predictions that ordered magnetospheres can produce non-thermal, quiescent radio emission across a wide mass range—from massive magnetic stars to brown dwarfs and fully convective M dwarfs. Correlations between quiescent radio and Balmer emission seen in other systems further suggest aurora-linked magnetospheric processes rather than chromospheric activity. Variability on days-long timescales, as observed here and in Solar System belts, may reflect changes in radial diffusion or magnetospheric reconfiguration. Potential plasma sources include volcanically active satellites or planets (by analogy with Io) and seed electrons from stellar-like flares that are subsequently accelerated by rotationally driven processes. Overall, the detection evidences a common magnetospheric framework for non-thermal radio emission beyond the Solar System.

This work presents the first resolved imaging that confirms a radiation-belt-like structure around the ultracool dwarf LSR J1835+3259. The stable, double-lobed synchrotron morphology, large spatial extent (up to ~18 RUCD), and inferred ~15 MeV electron energies establish an extrasolar analogue to Jovian radiation belts and demonstrate that rotating dipolar magnetospheres can produce persistent, non-thermal radio emission in ultracool dwarfs. The findings validate predictions of radiation belts at both extremes of the stellar mass sequence and motivate a broader re-examination of quiescent radio emission mechanisms in brown dwarfs, fully convective M dwarfs, and massive magnetic stars. Future work should obtain linear polarization measurements to conclusively confirm synchrotron emission, conduct multi-frequency, multi-epoch imaging to map energy-dependent belt structure and variability, and search for satellites or planets that could supply magnetospheric plasma. Improved sensitivity and baseline coverage will help trace fainter, more extended emission and better constrain magnetic topology and particle acceleration processes.

• Linear polarization was not calibrated; thus, the synchrotron interpretation lacks direct linear polarization confirmation.
• Missing antennas and challenging long-baseline calibration (e.g., MK–EB) reduced sensitivity to extended structures and required excluding some baselines/antennas in final imaging.
• Major axes of individual lobes were unresolved in fits; faint, more extended emission could be undetected in some epochs, limiting size estimates.
• Only three epochs at a single frequency (8.4 GHz) were used for resolved imaging; frequency-dependent morphology is inferred via external contemporaneous results rather than directly imaged here.
• Instrumental circular polarization initially at 7–10% required careful calibration; residual systematics at the ~1% level may remain.
• The estimated synchrotron cooling time (~60 days) versus year-long persistence implies ongoing acceleration or replenishment not directly observed; the plasma source (e.g., flares vs. satellites) remains uncertain.