Deep Images of the Galactic Center with GRAVITY
  Stellar orbits at the Galactic Center provide a very clean probe of the
gravitational potential of the supermassive black hole. They can be studied
with unique precision, beyond the confusion limit of a single telescope, with
the near-infrared interferometer GRAVITY. Imaging is essential to search the
field for faint, unknown stars on short orbits which potentially could
constrain the black hole spin. Furthermore, it provides the starting point for
astrometric fitting to derive highly accurate stellar positions. Here, we
present $\mathrm{G^R}$, a new imaging tool specifically designed for Galactic
Center observations with GRAVITY. The algorithm is based on a Bayesian
interpretation of the imaging problem, formulated in the framework of
information field theory and building upon existing works in
radio-interferometric imaging. Its application to GRAVITY observations from
2021 yields the deepest images to date of the Galactic Center on scales of a
few milliarcseconds. The images reveal the complicated source structure within
the central $100\,\mathrm{mas}$ around Sgr A*, where we detected the stars S29
and S55 and confirm S62 on its trajectory, slowly approaching Sgr A*.
Furthermore, we were able to detect S38, S42, S60, and S63 in a series of
exposures for which we offset the fiber from Sgr A*. We provide an update on
the orbits of all aforementioned stars. In addition to these known sources, the
images also reveal a faint star moving to the west at a high angular velocity.
We cannot find any coincidence with any known source and, thus, we refer to the
new star as S300. From the flux ratio with S29, we estimate its K-band
magnitude as $m_\mathrm{K}\left(\mathrm{S300}\right)\simeq 19.0 - 19.3$. Images
obtained with CLEAN confirm the detection.
