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We have resolved the newly discovered T dwarf SDSS J1534+1615 into a 0.11''binary using the Keck sodium laser guide star adaptive optics system. With anintegrated-light spectral type of T3.5, this binary provides a new benchmarkfor studying the distinctive J-band brightening previously noted among earlyand mid-T dwarfs, using two brown dwarfs with different spectral types buthaving a common metallicity and age and very similar surface gravities. Weestimate spectral types of T1.5+/-0.5 and T5.5+/-0.5 for the two componentsbased on their near-IR colors, consistent with modeling the integrated-lightspectrum as the blend of two components. The observed near-IR flux ratios areunique compared to all previously known substellar binaries: the component thatis fainter at H and K' is brighter at J. This inversion of the near-IR fluxesis a manifestation of the J-band brightening within this individual binarysystem. Therefore, SDSS 1534+1615 demonstrates that the brightening can beintrinsic to ultracool photospheres (e.g., arising from cloud disruption and/orrapid increase in cloud sedimentation) and does not necessarily result fromphysical variations among the observed ensemble of T dwarfs (e.g., a range inmasses, ages and/or metallicities). We suggest that the apparently largeamplitude of the J-band brightening may be due to a high incidence ofunresolved binaries and that the true amplitude of the phenomenon could be moremodest. This scenario would imply that truly single objects in these spectralsubclasses are relatively rare, in agreement with the small effectivetemperature range inferred for the L/T transition.Comment: ApJ, in press, 26 page

SDSS J1534+1615AB: A Novel T Dwarf Binary Found with Keck Laser Guide Star Adaptive Optics and the Potential Role of Binarity in the L/T Transition