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

Millimeter very long baseline interferometry (mm-VLBI) provides thenovel capacity to probe the emission region of a handful of supermassiveblack holes on sub-horizon scales. For Sagittarius A* (Sgr A*), thesupermassive black hole at the center of the Milky Way, this providesaccess to the region in the immediate vicinity of the horizon. Brodericket al. have already shown that by leveraging spectral and polarizationinformation as well as accretion theory, it is possible to extractaccretion-model parameters (including black hole spin) from mm-VLBIexperiments containing only a handful of telescopes. Here we repeat thisanalysis with the most recent mm-VLBI data, considering a class ofaligned, radiatively inefficient accretion flow (RIAF) models. We findthat the combined data set rules out symmetric models for Sgr A*'s fluxdistribution at the 3.9σ level, strongly favoring length-to-widthratios of roughly 2.4:1. More importantly, we find that physicallymotivated accretion flow models provide a significantly better fit tothe mm-VLBI observations than phenomenological models, at the 2.9σlevel. This implies that not only is mm-VLBI presently capable ofdistinguishing between potential physical models for Sgr A*'s emission,but further that it is sensitive to the strong gravitational lensingassociated with the propagation of photons near the black hole. Basedupon this analysis we find that the most probable magnitude, viewingangle, and position angle for the black hole spin are a = 0.0^{+0.64+ 0.86}, \theta ={68^\circ }^{+5^\circ +9^\circ }_{-20^\circ-28^\circ }, and \xi ={-52^\circ }^{+17^\circ +33^\circ }_{-15^\circ-24^\circ } east of north, where the errors quoted are the 1σ and2σ uncertainties.