Testing General Relativity with Accretion-Flow Imaging of Sgr A*
  The Event Horizon Telescope is a global very-long baseline interferometer capable of probing potential deviations from the Kerr metric, which is believed to provide the unique description of astrophysical black holes. Here we report an updated constraint on the quadrupolar deviation of Sagittarius A* within the context of a radiatively inefficient accretion flow model in a quasi-Kerr background. We also simulate near-future constraints obtainable by the forthcoming eight-station array and show that in this model already a one-day observation can measure the spin magnitude to within $0.005$, the inclination to within $0.09^\circ$, the position angle to within $0.04^\circ$, and the quadrupolar deviation to within $0.005$ at $3\sigma$ confidence. Thus, we are entering an era of high-precision strong gravity measurements. 
Tim Johannsen, Carlos Wang, Avery E. Broderick, Sheperd S. Doeleman,   Vincent L. Fish, Abraham Loeb, Dimitrios Psaltis
astro-ph.HE astro-ph.GA gr-qc