  2MASSI J1315309-264951 is an L3 dwarf with strong H-alpha emission discovered
in the course of a color-selected survey for active galactic nuclei using the
Two-Micron All-Sky Survey (2MASS). The strength of its H-alpha emission
decreased by about a factor of two between two epochs separated by 137 days.
This is the first time that variable H-alpha emission has been reported in an L
dwarf, and is probably the first observation of an H-alpha flare in an L dwarf.
The value of log(L_Halpha/L_bol)>-4.17 observed at the discovery epoch is
larger than that of any other L dwarf but comparable to that of 2MASSI
J1237392+652615, the only reported T dwarf with H-alpha emission. The observed
variability indicates that the H-alpha emission of 2MASSI J1315309-264951 is
powered either by magnetic fields or by accretion in a binary system.
Spectroscopic or narrow-band H-alpha monitoring of L and T dwarfs on timescales
of hours to days would be the most useful step toward a better understanding of
their H-alpha emission mechanism(s).
