Adaptive optics (AO) imaging and spectroscopy of the central star cluster in the Galactic Center over the past three decades have established that there is a concentration of 4 million solar masses associated with the compact radio source SgrA*, presumably a massive black hole. In 2017 we put into operation GRAVITY, for combining the near-IR light of all four 8m UT telescopes of the ESO-VLT for milli-arcsec imaging, and for improving the astrometric measurement precision tenfold compared to our previous AO data. DUring the peri-passage of the star S2 in May 2018 we were able to robustly detect the gravitational redshift and test Einstein's equivalence principle in the orbital elements of the star, thus for the first time testing GR in the high mass regime. The detection of the Schwarzschild pression of the orbit is expected in the next year. During bright near-IR 'flares' SgrA* exhibts 100 micro-arcsec loops/wobbles, which may be interpreted as orbital motion of 'hot spots' in the accretion flow on scales of 4-5 R_S. If so, the mass within the hot spot orbits is consistent with the 4 million solar masses. The flares also exhibit polarization loops and we find that their orbital angular momentum is close to that of the OB-star disk at 10^5 R_S. I will end with an outlook how measurements with GRAVITY and the EHT can together test the Kerr metric on scales of a few R_S.
Speaker: Reinhard Genzel, Max-Planck Institute for Extraterrestrial Physics
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