title: Testing General Relativity with the Shadow Size of Sgr A(*).
abstract: 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.
authors: Perimeter Institute for Theoretical Physics, Waterloo, Ontario N2L 2Y5, Canada.|Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
authors: Tim
authors: T
authors: Johannsen
authors: Perimeter Institute for Theoretical Physics, Waterloo, Ontario N2L 2Y5, Canada.|Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
authors: Avery E
authors: AE
authors: Broderick
authors: Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany.
authors: Philipp M
authors: PM
authors: Plewa
authors: Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany.
authors: Sotiris
authors: S
authors: Chatzopoulos
authors: MIT Haystack Observatory, Westford, Massachusetts 01886, USA.|Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA.
authors: Sheperd S
authors: SS
authors: Doeleman
authors: Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany.
authors: Frank
authors: F
authors: Eisenhauer
authors: MIT Haystack Observatory, Westford, Massachusetts 01886, USA.
authors: Vincent L
authors: VL
authors: Fish
authors: Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany.|Physics and Astronomy Departments, University of California, Berkeley, California 94720, USA.
authors: Reinhard
authors: R
authors: Genzel
authors: Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany.
authors: Ortwin
authors: O
authors: Gerhard
authors: Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA.
authors: Michael D
authors: MD
authors: Johnson