Discovery of a T dwarf binary with the largest known J-band flux reversal

We present Keck laser guide star observations of two T2.5 dwarfs-2MASS J11061197+2754225 and 2MASS J14044941-3159329-using NIRC2 on Keck II and find 2MASS J14044941-3159329 to be a 0.13 '' binary. This system has a secondary that is 0.45 mag brighter than the primary in J band, but 0.49 mag fainter in H band and 1.13 mag fainter in K(s) band. We use this relative photometry along with near-infrared synthetic modeling performed on the integrated light spectrum to derive component types of T1 +/-1 for the primary and T5 +/-1 for the secondary. Optical spectroscopy of this system obtained with Magellan/LDSS-3 is also presented. This is the fourth L/T transition binary to show a flux reversal in the 1-1.2 mu m regime, and this one has the largest flux reversal. Unless the secondary is itself an unresolved binary, the J-band magnitude difference between the secondary and primary shows that the J-band bump'' is indeed a real feature and not an artifact caused by unresolved binarity.