  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 is
coincident to within 18 mas of the Galaxy's central supermassive black hole and
the unique radio source Sgr A*. While in 2002 this source (SgrA*-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 flux density of 0.7 - 3 mJy, observed, and 4 - 17 mJy,
dereddened; 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 min. 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 micron flux density variations
implies that the emission arises in the accretion flow on physical size scales
smaller than 5 AU, or 80 R_s for a 4x10^6 Mo black hole. We suggest that the
3.8 micron emission and the X-ray flares arise from the same underlying
physical process, possibly the acceleration of a small populations of electrons
to ultrarelativistic energies. In contrast to the X-ray flares which are only
detectable 2% of the time, the 3.8 micron emission provides a new, constantly
accessible, window into the physical conditions of the plasma in close
proximity to the central black hole.

Astrophysics