Resolved imaging confirms a radiation belt around an ultracool dwarf

Radiation belts are a hallmark feature of planetary magnetospheres in our solar system, encompassing Earth, Jupiter, Saturn, Uranus, and Neptune. These equatorial zones contain relativistic particles reaching energies of tens of megaelectron volts, extending to more than ten planetary radii. They emit gradually varying radio emissions, influence the surface chemistry of nearby moons, and are intrinsically linked to magnetospheric processes. Recent observations have indicated that ultracool dwarfs (UCDs), encompassing very low-mass stars and brown dwarfs, can produce planet-like radio emissions, including periodically bursting aurorae stemming from large-scale magnetospheric currents and slowly varying quiescent radio emissions potentially linked to low-level coronal flaring. However, these quiescent emissions deviate from established multiwavelength flare relationships, prompting further investigation. This study focuses on resolving the origin and nature of these quiescent radio emissions in UCDs, aiming to understand if they are analogous to the radiation belts observed in our solar system.

Prior research has demonstrated the existence of planet-like radio emissions from ultracool dwarfs, including periodic auroral bursts and slowly varying quiescent emission. The auroral bursts are understood to originate from magnetospheric currents and are associated with strong magnetic fields. The quiescent emission, however, has been more enigmatic, with hypotheses ranging from low-level coronal flaring to other magnetospheric processes. Existing models have attempted to explain this emission using various mechanisms, but discrepancies remain, particularly concerning the lack of adherence to standard multiwavelength flare relationships observed in more massive stars. This study builds upon these earlier findings, aiming to provide a clearer understanding of the source of the quiescent radio emission and its relationship to planetary radiation belts.

The research employed the High Sensitivity Array (HSA), a network of 39 radio dishes across the USA and Germany, to image the ultracool dwarf LSR J1835+3259 at 8.4 GHz. Three five-hour observation epochs were conducted between 2019 and 2020, capturing nearly two full rotation periods per epoch. Data reduction involved standard very long baseline interferometry techniques using the Astronomical Image Processing System (AIPS). The team meticulously addressed challenges such as missing antenna data in some epochs and corrected for the target's proper motion and parallax using Gaia Data Release 3. Careful calibration was performed to minimize instrumental polarization, allowing for a reliable analysis of circular and potentially linear polarization from the source. Time series analysis was used to identify and remove auroral bursts, isolating the quiescent emission for imaging and analysis. Electron energies were estimated using the observed frequency and the characteristics of synchrotron radiation.

The high-resolution imaging revealed that the quiescent radio emission from LSR J1835+3259 is spatially resolved and exhibits a stable, double-lobed morphology extending up to 18 ultracool dwarf radii across three observations spanning over a year. The double-lobed structure is remarkably similar in morphology to the Jovian radiation belts. The auroral bursts appear centrally located between the two lobes in the highest-quality data epoch. Analysis of the lobe separation, considering a surface dipole field of at least 3 kG, suggests electron energies of approximately 15 MeV, consistent with Jupiter's radiation belts. The lack of detectable circular polarization in the resolved radio lobes points towards synchrotron emission as the primary mechanism for the observed radiation. The 8.4 GHz quiescent emission is significantly more compact than the 4.5 GHz emission observed contemporaneously. The long synchrotron cooling time (estimated at 60 days) of the relativistic electrons, coupled with the persistence of the double-lobed structure for over a year, indicates that the emission is not solely attributable to stellar flares. The X-ray upper limit for LSR J1835+3259 is extremely low, suggesting minimal coronal heating from flaring activity.

The findings strongly support the existence of an extrasolar analog to planetary radiation belts, specifically those of Jupiter, around the ultracool dwarf LSR J1835+3259. The double-lobed, axisymmetric structure, electron energies, and lack of circular polarization in the radio emission are all highly consistent with this interpretation. The long-lived nature of the relativistic electron population challenges explanations based solely on stellar flares, suggesting other acceleration mechanisms are at play, such as those observed in Jupiter's magnetosphere, involving centrifugally outflowing plasma and rotationally driven currents. The results also align with recent radiation belt modeling for magnetized massive stars, highlighting the potential universality of this phenomenon across a wide range of stellar masses. The compact nature of the 8.4 GHz emission, compared to the 4.5 GHz emission, is consistent with higher-energy electrons dominating at higher frequencies, as observed in Jupiter's radiation belts.

This study provides compelling evidence for the existence of radiation belts around an ultracool dwarf, marking a significant advancement in our understanding of magnetospheric dynamics across the stellar mass sequence. The findings underscore the importance of considering alternative acceleration mechanisms beyond stellar flares in explaining quiescent radio emission from UCDs. Future research could focus on searching for planets or moons around LSR J1835+3259 that might seed the magnetosphere with plasma, and investigating the role of stellar flares in augmenting the electron population within the radiation belts.

The study is limited by the availability of data from the HSA. Missing antennas in some observation epochs affected the sensitivity and resolution of the imaging, particularly in Epoch 1. The absence of linear polarization calibrations prevented a definitive confirmation of synchrotron emission as the sole mechanism, although the lack of circular polarization strongly suggests its significant contribution. Further observations with more complete antenna coverage and linear polarization measurements are needed to fully characterize the emission.