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

TL;DR

This study detects orbital motions near the last stable circular orbit of the black hole SgrA* through infrared flares, revealing hot spot dynamics and polarization changes consistent with relativistic models.

Contribution

First direct observation of hot spot orbital motion near the innermost stable circular orbit of SgrA* using infrared interferometry and polarization measurements.

Findings

Detected centroid motion with ~30% light speed during flares

Observed polarization angle rotation with ~45-minute period

Modeling aligns with hot spot orbit near the ISCO of SgrA*

Abstract

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.