An Improved Distance and Mass Estimate for Sgr A* from a Multistar Orbit Analysis

We present new, more precise measurements of the mass and distance of our Galaxy’s central supermassive black hole, Sgr A*. These results stem from a new analysis that more than doubles the time baseline for astrometry of faint stars orbiting Sgr A*, combining 2 decades of speckle imaging and adaptive optics data. Specifically, we improve our analysis of the speckle images by using information about a star’s orbit from the deep adaptive optics data (2005─2013) to inform the search for the star in the speckle years (1995─2005). When this new analysis technique is combined with the first complete re-reduction of Keck Galactic Center speckle images using speckle holography, we are able to track the short-period star S0-38 (K-band magnitude = 17, orbital period = 19 yr) through the speckle years. We use the kinematic measurements from speckle holography and adaptive optics to estimate the orbits of S0-38 and S0-2 and thereby improve our constraints of the mass (M <SUB>bh</SUB>) and distance (R <SUB> o </SUB>) of Sgr A*: M <SUB>bh</SUB> = (4.02 ± 0.16 ± 0.04) × 10<SUP>6</SUP> M <SUB>☉</SUB> and 7.86 ± 0.14 ± 0.04 kpc. The uncertainties in M <SUB>bh</SUB> and R <SUB> o </SUB> as determined by the combined orbital fit of S0-2 and S0-38 are improved by a factor of 2 and 2.5, respectively, compared to an orbital fit of S0-2 alone and a factor of ∼2.5 compared to previous results from stellar orbits. This analysis also limits the extended dark mass within 0.01 pc to less than 0.13 × 10<SUP>6</SUP> M <SUB>☉</SUB> at 99.7% confidence, a factor of 3 lower compared to prior work.