1.3 mm WAVELENGTH VLBI OF SAGITTARIUS A*: DETECTION OF TIME-VARIABLE EMISSION ON EVENT HORIZON SCALES
2011
Sagittarius A*, the ~4 × 10^6 M_⊙ black hole candidate at the Galactic center, can be studied on Schwarzschild
radius scales with (sub)millimeter wavelength very long baseline interferometry (VLBI). We report on 1.3 mm
wavelength observations of Sgr A* using a VLBI array consisting of the JCMT on Mauna Kea, the Arizona Radio
Observatory’s Submillimeter Telescope on Mt. Graham in Arizona, and two telescopes of the CARMA array at
Cedar Flat in California. Both Sgr A* and the quasar calibrator 1924−292 were observed over three consecutive
nights, and both sources were clearly detected on all baselines. For the first time, we are able to extract 1.3mmVLBI
interferometer phase information on Sgr A* through measurement of closure phase on the triangle of baselines. On
the third night of observing, the correlated flux density of Sgr A* on all VLBI baselines increased relative to the
first two nights, providing strong evidence for time-variable change on scales of a few Schwarzschild radii. These
results suggest that future VLBI observations with greater sensitivity and additional baselines will play a valuable
role in determining the structure of emission near the event horizon of Sgr A*.
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