Detection of the Intrinsic Size of Sagittarius A* Through Closure Amplitude Imaging

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 +/- 2 Schwarzschild radii at 7-millimeter wavelength. In one of eight 7-millimeter epochs, we also detected an increase in the intrinsic size of 60⁺²⁵<INF ARRANGE="STAGGER">-17%. These observations place a lower limit to the mass density of Sagittarius A* of 1.4 × 10⁴ solar masses per cubic astronomical unit.