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

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Abstract
Residing in the centre of the Milky Way, Sagittarius A* (Sgr A*) is the closest massive black hole 1 (MBH). Its vicinity has allowed measuring individual stellar orbits around it 2–4 . The stars act as test particles and probe the gravitational potential around the 4.3 × 10 6 M ⊙ MBH. These observations have determined the central mass to sub-per-cent precision 5 , and the mildly relativistic motions of stars have given access to the dominant relativistic corrections, the gravitational redshift 6,7 , the transverse Doppler effect and the prograde precession imposed by the Schwarzschild metric nature of the potential 8 . These effects are of order β 2 = ( v / c ) 2 (for velocity v and speed of light c ). The Kerr metric for a rotating black hole leads to corrections of order β 3 . Here, we report the discovery of a faint main-sequence star ( m K = 19.3), S301, on an 8.7-year orbit and with small enough a pericentre distance, such that the peak velocity of the star reaches 25,000 km s −1 . Within the measurement abilities of current near-infrared interferometry and future spectroscopy on an extremely large telescope, the motion of S301 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 by the Hills mechanism.

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