WISEP J061135.13-041024.0 AB: A J-band Flux Reversal Binary at the L/T Transition

We present Keck II laser guide star adaptive optics observations of the brown dwarf WISEP J061135.13−041024.0 showing it is a binary with a component separation of 0. 4. This system is one of the six known resolved binaries in which the magnitude differences between the components show a reversal in sign between the Y/J band and the H/K bands. Deconvolution of the composite spectrum results in a best-fit binary solution with L9 and T1.5 components. We also present a preliminary parallax placing the system at a distance of 21.2 ± 1.3 pc. Using the distance and resolved magnitudes we are able to place WISEP J061135.13−041024.0 AB on a color–absolute magnitude diagram, showing that this system contributes to the well-known “J-band bump” and the components’ properties appear similar to other late-type L and early-type T dwarfs. Fitting our data to a set of cloudy atmosphere models suggests the system has an age >1 Gyr with WISE 0611−0410 A having an effective temperature (Teff) of 1275–1325 K and mass of 64–65 MJup, and WISE 0611−0410 B having Teff = 1075–1115 K and mass 40–65 MJup.