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Millimeter very-long baseline interferometry (mm-VLBI) provides the novelcapacity to probe the emission region of a handful of supermassive black holeson sub-horizon scales. For Sagittarius A* (Sgr A*), the supermassive black holeat the center of the Milky Way, this provides access to the region in theimmediate vicinity of the horizon. Broderick et al. (2009) have already shownthat by leveraging spectral and polarization information as well as accretiontheory, it is possible to extract accretion-model parameters (including blackhole spin) from mm-VLBI experiments containing only a handful of telescopes.Here we repeat this analysis with the most recent mm-VLBI data, considering aclass of aligned, radiatively inefficient accretion flow (RIAF) models. We findthat the combined data set rules out symmetric models for Sgr A*'s fluxdistribution at the 3.9-sigma level, strongly favoring length-to-width ratiosof roughly 2.4:1. More importantly, we find that physically motivated accretionflow models provide a significantly better fit to the mm-VLBI observations thanphenomenological models, at the 2.9-sigma level. This implies that not only ismm-VLBI presently capable of distinguishing between potential physical modelsfor Sgr A*'s emission, but further that it is sensitive to the stronggravitational lensing associated with the propagation of photons near the blackhole. Based upon this analysis we find that the most probable magnitude,viewing angle, and position angle for the black hole spin area=0.0(+0.64+0.86), theta=68(+5+9)(-20-28) degrees, and xi=-52(+17+33)(-15-24)east of north, where the errors quoted are the 1-sigma and 2-sigmauncertainties.

Evidence for Low Black Hole Spin and Physically Motivated Accretion Models from Millimeter VLBI Observations of Sagittarius A*