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

Abstract

Ultracool dwarfs exhibit radio emission revealing strong magnetic fields and complex magnetospheric processes. Their auroral emission resembles that 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 and very long baseline interferometry arrays with TESS optical photometry, spanning 2019-2023. We analyzed time-series data to derive precise rotation periods and constructed light curves and radio dynamic spectra. The radio lightcurves were modeled using an auroral framework analogous to Jupiter's magnetosphere, incorporating main oval and active field line components. We detect persistent, compact radio emission with significant variability and high circular polarization. Periodogram analysis yields a rotation period of 3.07941 $\pm$ 0.00020h from radio data, consistent with the TESS optical period (3.07908 $\pm$ 0.00038h). The radio light curves display stable morphology characterized by asymmetric, rotationally modulated pulses. Our modeling reproduces the emission using a hybrid auroral scenario, with the main oval contributing a stable component and an active field line producing dominant, variable bursts. Short-term variability is explained by small changes in emission cone geometry or 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 in a structured magnetosphere. The behavior is consistent with a Jupiter-like auroral system, possibly involving companion-driven or internally-driven magnetospheric interactions analogous to those observed in the Solar System.