Constraining a Companion of the Galactic Center Black Hole Sgr A

We use 23 yr of astrometric and radial velocity data on the orbit of the star S0-2 to constrain a hypothetical intermediate-mass black hole orbiting the massive black hole Sgr A* at the Galactic center. The data place upper limits on variations of the orientation of the stellar orbit at levels between 002 and 007 per year. We use a combination of analytic estimates and full numerical integrations of the orbit of S0-2 in the presence of a black hole binary. For a companion intermediate-mass black hole outside the orbit of S0-2 (1020 au), we find that a companion black hole with mass m(c) between 10(3) and 10(5)M(circle dot) is excluded, with a boundary behaving as a(c)similar to m(c)(1/3). For a companion with a(c) < 1020 au, a black hole with mass between 10(3) and 10(5)M(circle dot) is excluded, with a(c)similar to m(c)(-1/2). These bounds arise from quadrupolar perturbations of the orbit of S0-2. Significantly stronger bounds on an inner companion arise from the fact that the location of S0-2 is measured relative to the bright emission of Sgr A* and that separation is perturbed by the "wobble" of Sgr A* about the center of mass between it and the companion. The result is a set of bounds as small as 400 M-circle dot at 200 au; the numerical simulations suggest a bound from these effects varying as a(c)similar to m(c)(-1). We compare and contrast our results with those from a recent analysis by the GRAVITY collaboration.