2025A&A...702A...1N

SIMP-0136 is a T2.5 brown dwarf whose young age (200 ± 50 Myr) and low mass (15 ± 3 M_Jup) make it an ideal analogue for the directly imaged exoplanet population. With a 2.4 hour period, it is known to be variable in both the infrared (IR) and the radio, which has been attributed to changes in the cloud coverage and the presence of an aurora, respectively. To quantify the changes in the atmospheric state that drive this variability, we obtained time-series spectra of SIMP-0136 covering one full rotation with both NIRSpec/PRISM and the MIRI/LRS on board JWST. We performed a series of time-resolved atmospheric retrievals using petitRADTRANS to measure changes in the temperature structure, chemistry, and cloudiness. We inferred the presence of a ∼250 K thermal inversion above 10 mbar of SIMP-0136 at all phases and we propose that this inversion is due to the deposition of energy into the upper atmosphere by an aurora. Statistical tests were performed to determine which parameters were driving the observed spectroscopic variability. The primary contribution was due to changes in the temperature profile at pressures deeper than 10 mbar, which resulted in variation of the effective temperature from 1243 K to 1248 K. This changing effective temperature was also correlated to observed changes in the abundances of CO₂ and H₂S, while all other chemical species were consistent with being homogeneous throughout the atmosphere. Patchy silicate clouds were required to fit the observed spectra, but the cloud properties were not found to systematically vary with longitude. This work paints a portrait of an L-T transition object, where the primary variability mechanisms are magnetic and thermodynamic in nature, rather than due to inhomogeneous cloud coverage.