Detection of faint stars near SgrA* with GRAVITY
  The spin of the supermassive black hole that resides at the Galactic Centre
can in principle be measured by accurate measurements of the orbits of stars
that are much closer to SgrA* than S2, the orbit of which recently provided the
measurement of the gravitational redshift and the Schwarzschild precession. The
GRAVITY near-infrared interferometric instrument combining the four 8m
telescopes of the VLT provides a spatial resolution of 2-4 mas, breaking the
confusion barrier for adaptive-optics-assisted imaging with a single 8-10m
telescope. We used GRAVITY to observe SgrA* over a period of six months in 2019
and employed interferometric reconstruction methods developed in radio
astronomy to search for faint objects near SgrA*. This revealed a slowly moving
star of magnitude 18.9 in K band within 30mas of SgrA*. The position and proper
motion of the star are consistent with the previously known star S62, which is
at a substantially larger physical distance, but in projection passes close to
SgrA*. Observations in August and September 2019 easily detected S29, with
K-magnitude of 16.6, at approximately 130 mas from SgrA*. The planned upgrades
of GRAVITY, and further improvements in the calibration, hold the promise of
finding stars fainter than magnitude 19 at K.
