Pinpointing the near-infrared location of Sgr A* by correcting optical distortion in the NACO imager

Near-infrared observations of stellar orbits at the Galactic Center provide conclusive evidence for a massive black hole associated with the compact radio source Sgr A*. The astrometric reference frame for these observations is tied to a set of red giant stars, which are also detectable at radio wavelengths through SiO maser emission in their envelopes. We have improved the precision and long-term stability of this reference frame, in which Sgr A* is localized to within a factor 5 better than previously: similar to 0.17 mas in position (in 2009) and similar to 0.07 mas yr(-1) in velocity. This improvement is the result of modelling and correcting optical distortion in the VLT/NACO imager to a submas level and including new infrared and radio measurements, which now both span more than a decade in time. A further improvement will follow future observations and facilitate the detection of relativistic orbital effects.