Discovery of a star sensitive to the spin of Sgr A*

Residing in the center of the Milky Way, Sgr A* is the closest massive black hole. Its vicinity has allowed measuring individual stellar orbits around it with periods as short as 12 years. The stars act as test particles and probe the gravitational potential around the 4.3 × 10^6 M_sun black hole. These observations have determined the central mass to sub-percent precision, and the mildly relativistic motions of stars have given access to seeing the dominant relativistic corrections, the gravitational redshift, the transverse Doppler effect and the prograde precession imposed by the Schwarzschild metric nature of the potential. These effects are of order (v/c)^2 (for velocity v and speed of light c). The Kerr metric occur- ring for a rotating black hole leads to corrections of order (v/c)^3. Here, we report the discovery of a faint main sequence star (mK = 19.3), S301, the orbit of which has a period of 8.7 years and small enough a pericenter distance, such that the star’s peak velocity reaches 25000 km/s or 8% of c. Within the measurement capabilities of current near-infrared interferometry and future spectroscopy on an extremely large telescope, S301’s motion is directly sensitive to the spin of Sgr A*. The high eccentricity of S301 suggests that it is the captured component of a binary that was torn apart via the Hills mechanism, sending the other component away as a hyper-velocity star.