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We report the first comprehensive study of emission line variability in anaccreting young brown dwarf. We have collected 14 high-resolution opticalspectra of 2MASSW J1207334-393254 (M8), a likely member of the nearby8-million-year-old TW Hydrae association. These spectra show a variety ofemission lines that are commonly seen in classical T Tauri stars. Halpha linein particular shows dramatic changes in shape and intensity in our dataset,both on timescales of several weeks and several hours. In spectra fromlate-January, the line is relatively weak and only slightly asymmetric. Spectrafrom mid- and late-March show intense, broad and asymmetric Halpha emission,indicative of on-going disk accretion. We estimate that the accretion ratecould have changed by a factor of 5-10 over ~6 weeks in this brown dwarf. Marchspectra also reveal significant `quasi-periodic' changes in the Halpha lineprofile over the course of a night, from clearly double-peaked to nearlysymmetric. These nightly profile changes, roughly consistent with the browndwarf's rotation period, could be the result of a redshifted absorption featurecoming into and out of our line of sight; when the profile is double-peaked wemay be looking into an accretion column, flowing from the inner disk edge on tothe central object, indicating that the accretion is probably channelled alongthe magnetic field lines. Our findings provide strong support for themagnetospheric accretion scenario, and thus for the existence of large-scalemagnetic fields, in the sub-stellar regime (abridged).Comment: 12 pages, 2 figures, ApJL, accepte

Whims of an Accreting Young Brown Dwarf: Exploring the Emission-Line Variability of 2MASSW J1207334-393254