Detection of the Intrinsic Size of Sagittarius A* through Closure   Amplitude Imaging (includes Online Supporting Material)
  We have detected the intrinsic size of Sagittarius A*, the Galactic Center radio source associated with a supermassive black hole, showing that the short-wavelength radio emission arises from very near the event horizon of the black hole. Radio observations with the Very Long Baseline Array show that the source has a size of $24 \pm 2$ Schwarzschild radii at 7 mm wavelength. In one of eight 7-mm epochs we also detect an increase in the intrinsic size of $60^{+25}_{-17}$%. These observations place a lower limit to the mass density of Sgr A* of $1.4\times10^{4}$ solar masses per cubic astronomical unit. 
Geoffrey C. Bower, Heino Falcke, Robeson M. Herrnstein, Jun-Hui Zhao,   W.M. Goss, & Donald C. Backer