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Phys Rev Lett. 2016 Jan 22;116(3):031101. doi: 10.1103/PhysRevLett.116.031101. Epub 2016 Jan 19.

Testing General Relativity with the Shadow Size of Sgr A(*).

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Perimeter Institute for Theoretical Physics, Waterloo, Ontario N2L 2Y5, Canada.
Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany.
MIT Haystack Observatory, Westford, Massachusetts 01886, USA.
Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA.
Physics and Astronomy Departments, University of California, Berkeley, California 94720, USA.


In general relativity, the angular radius of the shadow of a black hole is primarily determined by its mass-to-distance ratio and depends only weakly on its spin and inclination. If general relativity is violated, however, the shadow size may also depend strongly on parametric deviations from the Kerr metric. Based on a reconstructed image of Sagittarius A^{*} (Sgr A^{*}) from a simulated one-day observing run of a seven-station Event Horizon Telescope (EHT) array, we employ a Markov chain Monte Carlo algorithm to demonstrate that such an observation can measure the angular radius of the shadow of Sgr A^{*} with an uncertainty of ∼1.5  μas (6%). We show that existing mass and distance measurements can be improved significantly when combined with upcoming EHT measurements of the shadow size and that tight constraints on potential deviations from the Kerr metric can be obtained.

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