A 2MASS Ultracool M Dwarf Observed in a Spectacular Flare

2MASSW J0149090+295613 (hereafter 2M0149) is unique among the very late M/L dwarf counterparts to Two Micron All Sky Survey (2MASS) point sources in having shown, in the first set of spectra taken on 1997 December 7, a diverse emission-line spectrum at red wavelengths, which features an Halpha equivalent width of 300 Å. On four repeat observations, however, the spectrum was that of a more ordinary dMe object, albeit with a late spectral type of M9.5 V. Our interpretation is that the object underwent an extreme magnetic flare event. The continuum flux at the short-wavelength end of the spectrum was also several times stronger during the apparent flare than in quiescence. Comparison is made with midtype M dwarfs of higher mass that have undergone strong flares, and with another known M9.5 emission-line object PC 0025+0047. The 2MASS object adds to the evidence that magnetic dynamo eruptions may release more energy relative to L_bol as the stellar mass decreases; if the total flare luminosity scales with Halpha in the same way as a well-observed event of the middle M dwarf AD Leo, 2M0149 may have had a flare luminosity that approached or exceeded its quiescent L_bol during the brief impulse phase. In contrast, its quiescent state shows an unremarkable level of chromospheric activity, which leads us to question whether it is unique at all; rather, it may be that many of the ultracool M and L dwarfs found by the Deep Near-Infrared Survey (DENIS) and 2MASS undergo similar flares.