Pinpointing the Massive Black Hole in the Galactic Center with Gravitationally Lensed Stars

Abstract

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 the IR emission from the BH, in order to constrain accretion models; for solving 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 fluctuations 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 intersection of these lines fixes its position. A joint search for a lensing signal and for the BH shows that the most likely point of intersection coincides with the center of acceleration of stars orbiting the BH. This statistical signal of lensing by the BH has a random probability of ~0.01. 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.