A search for variability in the active T dwarf 2MASS 1237+6526

We present spectroscopic and imaging observations of the active T dwarf 2MASS 1237+6526, intended to investigate the emission mechanism of this cool brown dwarf. The Halpha emission line, first detected in 1999 July, appears to be persistent over 1.6 yr, with no significant variation from log(10) (LHalpha/L-bol) = -4.3, ruling out flaring as a possible source. The relatively high level of emission in this object appears to be unique among observed late-L and T dwarfs. One of our spectra shows an apparent velocity shift in the Halpha line, which could indicate an accretion hot spot in orbit around the brown dwarf; further confirmation of this shift is required. J-band monitoring observations fail to detect any significant variability (e.g., eclipsing events) at the +/-0.025 mag level over periods of up to 2.5 hr, and there appears to be no statistical evidence of variability for periods of up to 14 hr. These limits constrain the mass of a hypothetical interacting secondary to M-2 less than or similar to 20 M-Jup for inclinations i greater than or similar to 60degrees. While our observations do not explicitly rule out the binary hypothesis for this object, it does suggest that other mechanisms, such as youthful accretion, may be responsible.