Pinpointing the massive black hole in the galactic center with gravitationally lensed stars

A new statistical method for pinpointing the massive black hole (BH) in the Galactic center on the IR grid is presented and applied to astrometric IR observations of stars close to the BH. This is of interest for measuring th e IR emission from the BH, in order to constrain accretion models; for solv ing the orbits of stars near the BH, in order to measure the BH mass and to search for general relativistic effects; and for detecting the fluctuation s of the BH away from the dynamical center of the stellar cluster, in order to study the stellar potential. The BH lies on the line connecting the two images of any background source it gravitationally lenses, and so the inte rsection of these lines fixes its position. A joint search for a lensing si gnal and for the BH shows that the most likely point of intersection coinci des with the center of acceleration of stars orbiting the BH. This statisti cal signal of lensing by the BH has a random probability of similar to0.01. If true, it implies that there are a few distant supergiants behind the BH , while a simple mean Galactic model predicts none. This can be verified by deep IR stellar spectroscopy, which will determine whether the most likely lensed image pair candidates (listed here) have identical spectra.