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Sagittarius A*, the ~4 x 10^6 solar mass black hole candidate at the GalacticCenter, can be studied on Schwarzschild radius scales with (sub)millimeterwavelength Very Long Baseline Interferometry (VLBI). We report on 1.3 mmwavelength observations of Sgr A* using a VLBI array consisting of the JCMT onMauna Kea, the ARO/SMT on Mt. Graham in Arizona, and two telescopes of theCARMA array at Cedar Flat in California. Both Sgr A* and the quasar calibrator1924-292 were observed over three consecutive nights, and both sources wereclearly detected on all baselines. For the first time, we are able to extract1.3 mm VLBI interferometer phase information on Sgr A* through measurement ofclosure phase on the triangle of baselines. On the third night of observing,the correlated flux density of Sgr A* on all VLBI baselines increased relativeto the first two nights, providing strong evidence for time-variable change onscales of a few Schwarzschild radii. These results suggest that future VLBIobservations with greater sensitivity and additional baselines will play avaluable role in determining the structure of emission near the event horizonof Sgr A*.

1.3 mm Wavelength VLBI of Sagittarius A*: Detection of Time-Variable Emission on Event Horizon Scales