We report the detection of a variable point source , imaged at L ’ ( 3.8 \mu m ) with the Keck II 10 m telescope ’ s adaptive optics system , that is coincident to within 18 mas ( 1 \sigma ) 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 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 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 \mu m flux density variations implies that the emission arises quite close to the black hole , within 5 AU , or 80 R _ { s } .
We suggest that both the variable 3.8 \mu m 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 \sim 2 % of the time , the 3.8 \mu m emission provides a new , constantly accessible , window into the physical conditions of the plasma in close proximity to the central black hole .