Detection of event-horizon-scale structure in Sgr A*

Our first observations of Sgr A* with a 3-station 1.3 mm wavelength VLBI array detected structure with a scale size of 37 micro arcseconds, or ~4 Schwarzschild radii of the 4 million solar mass black hole. The main result is that the emission region is too small to be a radiative surface enclosing the black hole. It is more likely a smaller region offset from the black hole, presumably in a compact portion of an accretion disk or jet that is Doppler-enhanced by its velocity along our line of sight.

Doeleman, et al; Nature, v 455 pp 78-80 (2008), "Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre"

Fitting the size of Sgr A* with 1.3 mm VLBI. A plot of correlated flux density vs. baseline length with squares showing the SMT-CARMA baseline, triangles showing the SMT-JCMT baseline, an an upper limit (arrow) for non-detections on the JCMT-CARMA baseline. Open symbols for April 10, 2007 and filled symbols for April 11. The solid curve is the circular Gaussian fit with FWHM of 43 micro arcseconds. Dashed line is for annular ring model of inner radius 35 micro arcseconds and outer radius of 80 micro arcseconds.
Map of the 1.3 mm VLBI array used to detect Sgr A*.