Intrinsic Size of Sagittarius A*: 72 Schwarzschild Radii

Abstract

Recent proper motion studies of stars at the very center of the Galaxy strongly suggest that Sagittarius A*, the compact nonthermal radio source at the Galactic center, is a 2.5×106 Msolar black hole. By means of near-simultaneous multiwavelength Very Long Baseline Array measurements, we determine for the first time the intrinsic size and shape of Sgr A* to be 72 Rsc(*) by less than 20 Rsc(*), with the major axis oriented essentially north-south, where Rsc(*) (≡7.5×1011 cm) is the Schwarzschild radius for a 2.5×106 Msolar black hole. Contrary to previous expectation that the intrinsic structure of Sgr A* is observable only at λ