Dr. Jonathan Weintroub, CfA 

"VLBI with the SMA: Probing the event horizon of Sgr A*"

Abstract:
A unique and challenging opportunity exists to observe the immediate
neighborhood of the super massive black hole in the center of our
galaxy (Sgr A*) at a resolution comparable to the size of its event
horizon. At a wavelength just below 1 mm the flux density from the
galactic center peaks, the emission is relatively unaffected by
turbulent scattering in the intervening plasma, and the opacity is
low.  At 0.8 mm and with 5000 km baselines the angular resolution of
an interferometer approaches 20 microarcseconds, the finest
resolution available in any astronomical waveband.

We are building a phased array processor and VLBI recording interface
which we plan to retrofit to the SMA in 2008.  This equipment will
allow the SMA to function as a high frequency VLBI station with its
full collecting area. In 2006 we installed a hydrogen maser time
standard at the SMA site. Fiber connections between the SMA and the
CSO and JCMT already exist, so those facilities can also participate
in phased array VLBI observations.  Thus the SMA has the potential to
become the center of submm wavelength VLBI on Mauna Kea, providing
the critical longest baselines.

In this talk I will make the case for submm VLBI observations of the
galactic center, and describe the planned instrumentation retrofit.
In April 2007 we observed at 1.3 mm with a 3-element ad-hoc VLBI
array. The JCMT reflector was used on Mauna Kea, with the SMA
generating the maser locked LO reference, and housing the Mark5b VLBI
data storage equipment. The other stations were the SMTO (Mount
Graham, AZ), and a single CARMA dish (Cedar Flat, CA).  I will
conclude by showing and discussing some initial detections from this
observation.