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We report the detection of continuous positional and polarization changes ofthe compact source SgrA* in high states ('flares') of its variable near-infrared emission with the near-infrared GRAVITY-Very Large TelescopeInterferometer (VLTI) beam-combining instrument. In three prominent brightflares, the position centroids exhibit clockwise looped motion on the sky, onscales of typically 150 micro-arcseconds over a few tens of minutes,corresponding to about 30% the speed of light. At the same time, the flaresexhibit continuous rotation of the polarization angle, with about the same45(+/-15)-minute period as that of the centroid motions. Modelling withrelativistic ray tracing shows that these findings are all consistent with anear face-on, circular orbit of a compact polarized 'hot spot' of infraredsynchrotron emission at approximately six to ten times the gravitational radiusof a black hole of 4 million solar masses. This corresponds to the region justoutside the innermost, stable, prograde circular orbit (ISCO) of aSchwarzschild-Kerr black hole, or near the retrograde ISCO of a highly spun-upKerr hole. The polarization signature is consistent with orbital motion in astrong poloidal magnetic field.

Detection of orbital motions near the last stable circular orbit of the massive black hole SgrA*