THE GALACTIC-CENTER RADIO-SOURCE SHINES BELOW THE COMPTON LIMIT

Absence of refractive scintillation of Sgr A*, the Galactic center radio source, at 1.3 and 0.8 mm wavelengths places an observational limit of brightness temperature T(b) less-than-or-similar-to 0.5 x 10(12) K on the source. This is less than the maximum brightness of an incoherently synchrotron-emitting plasma, 10(12) K, known as the Compton limit. The refractive scintillations expected, due to strong broadening of the source by the interstellar plasma, are observed by Zhao et al. at decimeter wavelengths. We fail to observe them at 1.3 and 0.8 mm wavelengths over time spans between 0.1 s and 24 hr. Such scintillations are quenched by source size greater than about 0.1 AU, or, equivalently, T(b) < 0.5 x 10(12) K for Sgr A*, at 0.8 mm wavelength. The scintillations would also require fluctuations with scale sizes of 0.1 AU in the scattering plasma, moving across the line of sight at velocities V perpendicular-to greater-than-or-similar-to 100 km s-1. We argue that the plasma that scatters Sgr A* satisfies the latter two conditions, and that the absence of scintillations is due to the size of the source. Our observation strengthens the identification of Sgr A* as a quiescent galactic nucleus.