2004ApJ...601L.159G

We report the detection of a variable point source, imaged at L' (3.8 µm) with the Keck II 10 m telescope's adaptive optics system, that is coincident to within 18 mas (1 σ) of the Galaxy's central supermassive black hole and the unique radio source Sgr A*. While in 2002 this source (Sgr A* IR) was confused with the stellar source S0-2, in 2003 these two sources are separated by 87 mas, allowing the new source's properties to be determined directly. On four separate nights, its observed L' magnitude ranges from 12.2 to 13.8, which corresponds to a dereddened flux density of 4-17 mJy; no other source in this region shows such large variations in flux density–a factor of 4 over a week and a factor of 2 over 40 minutes. In addition, it has a K-L' color greater than 2.1, which is at least 1 mag redder than any other source detected at L' in its vicinity. Based on this source's coincidence with the Galaxy's dynamical center, its lack of motion, its variability, and its red color, we conclude that it is associated with the central supermassive black hole. The short timescale for the 3.8 µm flux density variations implies that the emission arises quite close to the black hole, within 5 AU, or 80R_s. We suggest that both the variable 3.8 µm emission and the X-ray flares arise from the same underlying physical process, possibly the acceleration of a small population of electrons to ultrarelativistic energies. In contrast to the X-ray flares, which are only detectable ∼2% of the time, the 3.8 µm emission provides a new, constantly accessible window into the physical conditions of the plasma in close proximity to the central black hole.