Observing a Black Hole Event Horizon: (Sub)Millimeter VLBI of Sgr A*
  Very strong evidence suggests that Sagittarius A*, a compact radio source at
the center of the Milky Way, marks the position of a super massive black hole.
The proximity of Sgr A* in combination with its mass makes its apparent event
horizon the largest of any black hole candidate in the universe and presents us
with a unique opportunity to observe strong-field GR effects. Recent millimeter
very long baseline interferometric observations of Sgr A* have demonstrated the
existence of structures on scales comparable to the Schwarzschild radius. These
observations already provide strong evidence in support of the existence of an
event horizon. (Sub)Millimeter VLBI observations in the near future will
combine the angular resolution necessary to identify the overall morphology of
quiescent emission, such as an accretion disk or outflow, with a fine enough
time resolution to detect possible periodicity in the variable component of
emission. In the next few years, it may be possible to identify the spin of the
black hole in Sgr A*, either by detecting the periodic signature of hot spots
at the innermost stable circular orbit or parameter estimation in models of the
quiescent emission. Longer term, a (sub)millimeter VLBI "Event Horizon
Telescope" will be able to produce images of the Galactic center emission to
the see the silhouette predicted by general relativistic lensing. These
techniques are also applicable to the black hole in M87, where black hole spin
may be key to understanding the jet-launching region.
