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                              G C N E W S
                             * Newsflash *

            - The Newsletter for Galactic Center Research -

gcnews@mpifr-bonn.mpg.de             http://www.mpifr-bonn.mpg.de/gcnews
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Vol. 12, No. 12                                              Sep 26, 2000


Recently submitted papers:
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1) Enhanced Microlensing by Stars Around the Black Hole in the Galactic 
   Center (Alexander & Loeb, ApJ)
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Email    : tal@stsci.edu
Title    : Enhanced Microlensing by Stars Around the Black Hole in the 
           Galactic Center
Author(s): Tal Alexander(1) and Abraham Loeb(2)
Institute: (1) Institute for Advanced Study, Olden Lane, Princeton, 
           NJ08540., Present address: Space Telescope Science 
           Institute, 3700 San Martin Drive, Baltimore, MD 21218 (2) 
           Harvard-Smithsonian Center for Astrophysics, 60 Garden 
           Street,Cambridge,MA 02138
Paper    : ApJ submitted
EPrint   : astro-ph/0009404

Abstract:
The effect of stars on the lensing properties of the supermassive black 
hole in the Galactic Center is similar to the effect of planets on 
microlensing by a star. We show that the dense stellar cluster around 
Sgr A* enhances by up to an order of magnitude the probability of 
high-magnification lensing events of a distant background source by the 
black hole. Conversely, the gravitational shear of the black hole 
changes and enhances the microlensing properties of the individual 
stars. The effect is largest when the source image lies near the 
Einstein radius of the black hole ( 1.75^' ' +/- 0.20'' for a source at 
infinity). We estimate that the probability of observing at least one 
distant background star which is magnified by a factor >5 in any 
infrared snapshot of the inner 3'' of the Galactic Center is 20% with 
present-day photometric sensitivity. Deeper future photometry will 
significantly increase the number of detected images. The largest 
source of uncertainty in this estimate is the luminosity function of 
the background stars. The gravitational shear of the black hole 
lengthens the duration of high-magnification events near the Einstein 
radius up to a few months, and introduces a large variety of lightcurve 
shapes that are different from those of isolated microlenses. 
Identification of such events by image subtraction can be used to probe 
the mass function, density and velocity distributions of faint stars 
near the black hole, which are not detectable otherwise.
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    Angela Cotera                       Heino Falcke & Sera Markoff
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