Detection of orbital motions near the last stable circular orbit of the
  massive black hole SgrA*
  We report the detection of continuous positional and polarization changes of
the compact source SgrA* in high states ('flares') of its variable near-
infrared emission with the near-infrared GRAVITY-Very Large Telescope
Interferometer (VLTI) beam-combining instrument. In three prominent bright
flares, the position centroids exhibit clockwise looped motion on the sky, on
scales of typically 150 micro-arcseconds over a few tens of minutes,
corresponding to about 30% the speed of light. At the same time, the flares
exhibit continuous rotation of the polarization angle, with about the same
45(+/-15)-minute period as that of the centroid motions. Modelling with
relativistic ray tracing shows that these findings are all consistent with a
near face-on, circular orbit of a compact polarized 'hot spot' of infrared
synchrotron emission at approximately six to ten times the gravitational radius
of a black hole of 4 million solar masses. This corresponds to the region just
outside the innermost, stable, prograde circular orbit (ISCO) of a
Schwarzschild-Kerr black hole, or near the retrograde ISCO of a highly spun-up
Kerr hole. The polarization signature is consistent with orbital motion in a
strong poloidal magnetic field.
