2MASSI J1315309-264951:: An L dwarf with strong and variable Hα emission

The object 2MASSI J1315309-264951 is an L3 dwarf with strong H 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 Halpha emission decreased by about a factor of 2 between two epochs separated by 137 days. This is the first time that variable Halpha emission has been reported in an L dwarf, and it is probably the first observation of an Halpha flare in an L dwarf. The value of (L-Halpha/L-bol)> -4.17 at the discovery epoch is larger than that of any other L dwarf, but it is comparable to that of 2MASSI J1237392-652615, the only reported T dwarf with Halpha emission. The observed variability indicates that the H emission of 2MASSI J1315309-264951 is powered either by magnetic fields or by accretion in a binary system. Spectroscopic or narrowband Halpha monitoring of L and T dwarfs on timescales of hours to days would be the most useful step toward a better understanding of their Halpha emission mechanism(s).