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

GRAVITY Collaboration

Research output: Contribution to journalArticlepeer-review


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 μas 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) min 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.

Original languageEnglish
Article numberL10
JournalAstronomy and Astrophysics
StatePublished - 1 Oct 2018


  • Black hole physics
  • Galaxy: center
  • Gravitation
  • Relativistic processes


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