RADIO EMISSION AND ORBITAL MOTION FROM THE CLOSE-ENCOUNTER STAR–BROWN DWARF BINARY WISE J072003.20–084651.2

We report the detection of radio emission and orbital motion from the nearby star–brown dwarf binary WISE J072003.20–084651.2AB. Radio observations across the 4.5–6.5 GHz band with the Very Large Array identify at the position of the system quiescent emission with a flux density of 15 ± 3 μ Jy, and a highly polarized radio source that underwent a 2–3 minute burst with peak flux density 300 ± 90 μ Jy. The latter emission is likely a low-level magnetic flare similar to optical flares previously observed for this source. No outbursts were detected in separate narrow-band H α monitoring observations. We report new high-resolution imaging and spectroscopic observations that confirm the presence of a co-moving T5.5 secondary and provide the first indications of three-dimensional orbital motion. We used these data to revise our estimates for the orbital period (4.1 year) and tightly constrain the orbital inclination to be nearly edge-on (93 6 +1 6 −1 4 ), although robust measures of the component and system masses will require further monitoring. The inferred orbital motion does not change the high likelihood that this radio-emitting very low-mass binary made a close pass to the Sun in the past 100 kyr.