Intrinsic Size of Sgr a ? : 72 Schwarzschild Radiik

Recent proper motion studies of stars at the very center of the Galaxy strongly suggest that Sagittarius (Sgr) A ? , the compact nonthermal radio source at the Galactic Center, is a 2.510 6 M black hole. By means of near-simultaneous multi-wavelength Very Long Baseline Array measurements, we determine for the rst time the intrinsic size and shape of Sgr A ? to be 72 R sc (?) by < 20 R sc (?), with the major axis oriented essentially north-south, where R sc (?) (7:5 10 11 cm) is the Schwarzschild radius for a 2.510 6 M black hole. Contrary to previous expectation that the intrinsic structure of Sgr A ? is observable only at 1 mm, we can discern the intrinsic source size at 7 mm because (1) the scattering size along the minor axis is half that along the major axis, and (2) the near simultaneous multi-wavelength mapping of Sgr A ? with the same interferometer makes it possible to extrapolate precisely the minor axis scattering angle at 7 mm. The intrinsic size and shape place direct constraints on the various emission models for Sgr A ?. In particular, the advection dominated accretion ow model may have to incorporate a radio jet in order to account for the structure of Sgr A ? .