THE SIMULTANEOUS SPECTRUM OF SAGITTARIUS A-ASTERISK FROM 20 CENTIMETER TO 1 MILLIMETER AND THE NATURE OF THE MILLIMETER EXCESS

We report results of a multiwavelength campaign to measure the simulta neous spectrum of the supermassive black hole candidate Sgr A in the Galactic center from centimeter to millimeter wavelengths using the Ve ry Large Array, the Berkeley-Illinois-Maryland Array (BIMA), the Nobey ama 45 m, and the Institut de Radioastronomie Millimetrique (IRAM) 30 m telescopes. The observations confirm that the previously detected mi llimeter excess is an intrinsic feature of the spectrum of Sgr A. The excess can be interpreted as and effect of the presence of an ultraco mpact component of relativistic plasma with a size of a few Schwarzsch ild radii near the black hole. If so, Sgr A might offer a unique poss ibility to image the putative black hole against the background of thi s component with future millimeter VLBI experiments.