A determination of the mass of Sagittarius A * from its radio spectral and source size measurements

There is growing evidence that Sgr A* may be a million solar mass black hole accreting from the Galactic center wind. A consideration of the spectral and source size characteristics associated with this process can offer at least two distinct means of inferring the mass M, complementing the more traditional dynamical arguments. We show that M is unmistakably correlated with both the radio spectral index and the critical wave-length below which the intrinsic source size dominates over the angular broadening due to scattering in the interstellar medium. Current observations can already rule out a mass much in excess of 2 × 10⁶ M_⊙ and suggest a likely value close to 1 × 10⁶ M_⊙, in agreement with an earlier study matching the radio and highenergy spectral components. We anticipate that such a mass may be confirmed with the next generation of source-size observations using milliarcsecond angular resolution at 0.5-1 cm wavelengths.