2000AJ....120..473B

We report the detection of Hα emission in the T dwarf (methane brown dwarf) 2MASSW J1237392+652615 over three days using the Keck Low Resolution Imaging Spectrograph. The measured line flux, log (L_Hα/L_bol)=-4.3, is roughly consistent with early M dwarf activity levels and inconsistent with decreasing activity trends in late M and L dwarfs. Similar emission is not seen in two other T dwarfs. We speculate on several mechanisms that may be responsible for emission, including a strong magnetic field, continuous flaring, acoustic heat generation, and a close [a~(4-20)R_J] interacting binary, with the cooler component overflowing its Roche lobe. We suggest that the M9.5 Ve PC 0025+0447 could be a warm analog to 2MASS J1237+65 and may be powered by the latter mechanism.