2massi J1315309−264951: an L Dwarf with Strong and Variable Hα Emission

2MASSI J1315309−264951 is an L3 dwarf with strong Hα emission discovered in the course of a colorselected survey for active galactic nuclei using the Two-Micron All-Sky Survey (2MASS). The strength of its Hα emission decreased by about a factor of two between two epochs separated by 137 days. This is the first time that variable Hα emission has been reported in an L dwarf, and is probably the first observation of an Hα flare in an L dwarf. The value of log(LHα/Lbol) > −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 Hα emission. The observed variability indicates that the Hα emission of 2MASSI J1315309−264951 is powered either by magnetic fields or by accretion in a binary system. Spectroscopic or narrow-band Hα monitoring of L and T dwarfs on timescales of hours to days would be the most useful step toward a better understanding of their Hα emission mechanism(s). Subject headings: stars: activity, stars: low mass, brown dwarfs, stars: individual (2MASSI J1315309−264951, 2MASSI J1237392+652615, 2MASSWJ0036159+182110)