Falcke, Melia, Agol: Viewing the Shadow of the Black Hole at the Galactic Center
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Viewing the Shadow of the Black Hole at the Galactic Center

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Heino Falcke^{1}, Fulvio Melia^{2}, Eric Agol^{3}

^{1} Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany (hfalcke@mpifr-bonn.mpg.de)

^{2}Steward Observatory and Department of Physics, University of
Arizona, Tucson, AZ 85721 (melia@physics.arizona.edu)

^{3}Physics and Astronomy Department, Johns Hopkins University,
Baltimore, MD 21218 (agol@pha.jhu.edu)
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**Abstract:**

In recent years, the evidence for the existence of an ultra-compact
concentration of dark mass associated with the radio source Sgr A* in
the Galactic Center has become very strong. However, an unambiguous
proof that this object is indeed a black hole is still lacking. A
defining characteristic of a black hole is the event horizon. To a
distant observer, the event horizon casts a relatively large
``shadow'' with an apparent diameter of ~10 gravitational radii due to
bending of light by the black hole, nearly independent of the black
hole spin or orientation. The predicted size (~30 micro-arcseconds)
of this shadow for Sgr A* approaches the resolution of current
radio-interferometers. If the black hole is maximally spinning and
viewed edge-on, then the shadow will be offset by ~8 micro-arcseconds
from the center of mass, and will be slightly flattened on one side.
Taking into account scatter-broadening of the image in the
interstellar medium and the finite achievable telescope resolution, we
show that the shadow of Sgr A* may be observable with very
long-baseline interferometry at sub-millimeter wavelengths, assuming
that the accretion flow is optically thin in this region of the
spectrum. Hence, there exists a realistic expectation of imaging the
event horizon of a black hole within the next few years.

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Questions: Heino Falcke, hfalcke@mpifr-bonn.mpg.de