We report the detection of H \alpha 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 _ { H { \alpha } } /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 \sim 4 - 20 R _ { Jup } ) interacting binary , with the cooler component overflowing its Roche lobe .
We suggest that the M9.5Ve PC 0025+0447 could be a warm analogue to 2MASS J1237+65 , and may be powered by the latter mechanism .