A Jupiter-like radio aurora in the L3.5 dwarf LSPM J0036+1821

Abstract

Ultracool dwarfs exhibit magnetically driven radio emission that reveals the presence of strong magnetic fields and complex magnetospheric processes. Their auroral emission is similar to the emission observed in magnetized Solar System planets, providing a framework to investigate their magnetic topology. We aim to characterize the magnetic activity, rotation, and atmospheric variability of the L3.5 dwarf LSPM J0036+1821 through a multiwavelength analysis. We conducted a long-term monitoring campaign combining radio observations from the Very Large Array (VLA) and very long-baseline interferometry (VLBI) arrays with optical photometry from the Transiting Exoplanet Survey Satellite (TESS), spanning 2019--2023. We analyzed time-series data to derive precise rotation periods and constructed phase-folded light curves and radio dynamic spectra. The radio light curves were modeled using an auroral framework analogous to that for Jupiter’s magnetosphere, incorporating both main-oval and active field line components. We detected persistent compact radio emission with significant variability and high circular polarization. The periodogram analysis yielded a precise rotation period of 3.07941 ± 0.00020 h from radio data, which is consistent with the optical period derived from TESS (3.07908 ± 0.00038 h). The morphology of the radio light curves is stable and characterized by asymmetric rotationally modulated pulses. Our modeling successfully reproduced the observed emission using a hybrid auroral scenario, with the main-oval contributing a stable component and an active field line producing the dominant variable bursts. Short-term variability is explained by small changes in the emission cone geometry or in the longitude of the active field line. The radio emission of LSPM J0036+1821 is consistent with auroral processes driven by the electron-cyclotron maser instability (ECMI) in a structured magnetosphere. The observed behavior is consistent with a Jupiter-like auroral system and might involve companion-driven or interior-driven magnetospheric interactions analogous to those observed in the Solar System.