Resolved imaging confirms a radiation belt around an ultracool dwarf

Abstract

Radiation belts are present in all large-scale Solar System planetary magnetospheres: Earth, Jupiter, Saturn, Uranus and Neptune1. These persistent equatorial zones of relativistic particles up to tens of megaelectron volts in energy can extend further than ten times the planet’s radius, emit gradually varying radio emissions2–4 and affect the surface chemistry of close-in moons5. Recent observations demonstrate that very low-mass stars and brown dwarfs, collectively known as ultracool dwarfs, can produce planet-like radio emissions such as periodically bursting aurorae6–8 from large-scale magnetospheric currents9–11. They also exhibit slowly varying quiescent radio emissions7,12,13 hypothesized to trace low-level coronal flaring14,15 despite departing from empirical multiwavelength flare relationships8,15. Here we present high-resolution imaging of the ultracool dwarf LSR J1835 + 3259 at 8.4 GHz, demonstrating that its quiescent radio emission is spatially resolved and traces a double-lobed and axisymmetrical structure that is similar in morphology to the Jovian radiation belts. Up to 18 ultracool dwarf radii separate the two lobes, which are stably present in three observations spanning more than one year. For plasma confined by the magnetic dipole of LSR J1835 + 3259, we estimate 15 MeV electron energies, consistent with Jupiter’s radiation belts4. Our results confirm recent predictions of radiation belts at both ends of the stellar mass sequence8,16–19 and support broader re-examination of rotating magnetic dipoles in producing non-thermal quiescent radio emissions from brown dwarfs7, fully convective M dwarfs20 and massive stars18,21.

Peculiar Spectral Property of Coherent Radio Emission from a Hot Magnetic Star: The Case of an Extreme Oblique Rotator

“…With an upper cutoff at ≈3 GHz, and a polar magnetic field strength of ≈4.7 kG (Shultz et al 2019b, corresponding electron gyrofrequency is 13 GHz), HD 142990 reinforces the idea that premature upper cutoff frequency is a general characteristic of ECME produced by hot magnetic stars. In the future, it will be extremely important to investigate if the same is true for other objects with large-scale, ordered magnetic fields (e.g., UCDs), since, ECME upper cutoffs are often used as a measure of maximum surface magnetic field strengths in these cooler objects (e.g., Kao et al 2023).…”

A Radio Counterpart to a Jupiter-mass Binary Object in Orion

“…The objects with known radio emission that are more similar to the components of the JuMBOs are the ultracool dwarfs (Kao & Sebastian Pineda 2022). The radio emission from the latter objects has two main components: a periodically bursting emission arising in aurorae and a slowly varying, quiescent emission with a low degree of circular polarization arising from a structure similar in morphology to the Jovian radiation belts (Climent et al 2023;Kao et al 2023). The bursting component is circularly polarized and it is believed to be produced by the electron cyclotron maser instability (Hallinan et al 2006), the same coherent emission process that creates the auroral radio emission from the planets of the solar system (Pineda et al…”

“…On the other hand, the quiescent component is usually interpreted as being caused by radio emission from relativistic electrons giving rise to gyrosynchrotron or synchrotron mechanisms, according to how relativistic the electrons are. Now it is known that this quiescent emission is produced in radiation belts (Climent et al 2023;Kao et al 2023). Given that the radio emission from JuMBO 24 is steady in time, one would expect it to be produced by relativistic electrons spiraling in the magnetic fields of a radiation belt.…”

Galileo Observation of Electron Spectra Dawn‐Dusk Asymmetry in the Middle Jovian Magnetosphere: Evidence for Convection Electric Field

“…( M denotes M‐shell, the jovicentric radial distance of a field line at the magnetic equator normalized to 1 R J , based on the JRM09 internal magnetic field model combined with the current sheet model Con2020 (R. J. Wilson et al., 2023). ) Trapped at M < 5, such harsh electron radiation is the one in all within our reach strengthful enough for synchrotron emissions (Bolton et al., 2002) as the astrophysical sources (e.g., Kao et al., 2023). However, at ∼10 < M < 20 the magnetospheric configuration is ambiguous as it gradually transitions from a dipole‐ to sheet‐like structure.…”

“…Wilson et al, 2023).) Trapped at M < 5, such harsh electron radiation is the one in all within our reach strengthful enough for synchrotron emissions (Bolton et al, 2002) as the astrophysical sources (e.g., Kao et al, 2023). However, at ∼10 < M < 20 the magnetospheric configuration is ambiguous as it gradually transitions from a dipole-to sheetlike structure.…”