High-resolution VLBA imaging of the radio source Sgr A* at the Galactic Centre

Abstract

THE compact non-thermal radio source at the Galactic Centre, known as Sgr A*, may mark the location of a massive black hole1,2. Here we present images of Sgr A* with milliarcsecond resolution obtained by using five telescopes of the partially completed Very Long Baseline Array (VLBA) in conjunction with a few additional telescopes. The image of Sgr A* at a wavelength of 3.6 cm confirms almost exactly the elliptical gaussian model that has been proposed on the basis of previous, more sparse data3,4. The source size at 1.35 cm wavelength is 2.4±0.2 mas, similar to previous results3,5. At both wavelengths, the radio source is smooth, without detectable fine structure. These observations, along with other recent results6, support the suggestion7 that the radio emission from Sgr A* is strongly scattered by electron-density fluctuations along the line of sight. On the assumption8 that the emission is due to a black hole accreting stellar winds from massive stars in the central 0.5 pc, the observations are consistent with a black-hole mass of ≁2x106M⊙