# Pinpointing the massive black hole in the Galactic Center with gravitationally lensed stars

## Tal Alexander

(1) Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218

### Paper: ApJL, submitted

**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 combined 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 detection of lensing by the BH has a
random probability of 0.01. It can be verified by deep IR stellar
spectroscopy, which will determine whether the most likely lensed
image pair candidates (listed here) have identical spectra.

Preprints available from the authors at tal@stsci.edu
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