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.