Resolved imaging of an extrasolar radiation belt around an ultracool dwarf

Radiation belts are present in all large-scale Solar System planetary magnetospheres: Earth, Jupiter, Saturn, Uranus, and Neptune. These persistent equatorial zones of trapped high energy particles up to tens of MeV can produce bright radio emission and impact the surface chemistry of close-in moons. Recent observations confirm planet-like radio emission such as aurorae from large-scale magnetospheric current systems on very low mass stars and brown dwarfs. These objects, collectively known as ultracool dwarfs, also exhibit quiescent radio emission hypothesized to trace stellar coronal flare activity or extrasolar radiation belt analogs. Here we present high resolution imaging of the ultracool dwarf LSR J1835+3259 demonstrating that this radio emission is spatially resolved and traces a long-lived, double-lobed, and axisymmetric structure similar in morphology to the Jovian radiation belts. Up to 18 ultracool dwarf radii separate the two lobes. This structure is stably present in three observations spanning >1 year. We infer a belt-like distribution of plasma confined by the magnetic dipole of LSR J1835+3259, and we estimate ~15 MeV electron energies that are consistent with those measured in the Jovian radiation belts. Though more precise constraints require higher frequency observations, a unified picture where radio emissions in ultracool dwarfs manifest from planet-like magnetospheric phenomena has emerged. Submitted, under review.