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We report the detection of a variable point source, imaged at L'(3.8 microns)with the W. M. Keck II 10-meter telescope's adaptive optics system, that iscoincident to within 18 mas of the Galaxy's central supermassive black hole andthe unique radio source Sgr A*. While in 2002 this source (SgrA*-IR) wasconfused with the stellar source S0-2, in 2003 these two sources are separatedby 87 mas allowing the new source's properties to be determined directly. Onfour separate nights, its observed L' magnitude ranges from 12.2 to 13.8, whichcorresponds to a flux density of 0.7 - 3 mJy, observed, and 4 - 17 mJy,dereddened; no other source in this region shows such large variations in fluxdensity - a factor of 4 over a week and a factor of 2 over 40 min. In addition,it has a K-L' color greater than 2.1, which is at least 1 mag redder than anyother source detected at L' in its vicinity. Based on this source's coincidencewith the Galaxy's dynamical center, its lack of motion, its variability, andits red color, we conclude that it is associated with the central supermassiveblack hole. The short timescale for the 3.8 micron flux density variationsimplies that the emission arises in the accretion flow on physical size scalessmaller than 5 AU, or 80 R_s for a 4x10^6 Mo black hole. We suggest that the3.8 micron emission and the X-ray flares arise from the same underlyingphysical process, possibly the acceleration of a small populations of electronsto ultrarelativistic energies. In contrast to the X-ray flares which are onlydetectable 2% of the time, the 3.8 micron emission provides a new, constantlyaccessible, window into the physical conditions of the plasma in closeproximity to the central black hole.Comment: published in Astrophysical Journal Letter

Variable Infrared Emission from the Supermassive Black Hole at the Center of the Milky Way