Persistent Asymmetric Structure of Sagittarius A* on Event Horizon Scales
Creators: name: Fish, Vincent L.
name: Johnson, Michael D.
name: Doeleman, Sheperd S.
name: Broderick, Avery E.
name: Psaltis, Dimitrios
name: Lu, Ru-Sen
name: Akiyama, Kazunori
name: Alef, Walter
name: Algaba, Juan Carlos
name: Asada, Keiichi
name: Beaudoin, Christopher
name: Bertarini, Alessandra
name: Blackburn, Lindy
name: Blundell, Ray
name: Bower, Geoffrey C.
name: Brinkerink, Christiaan
name: Cappallo, Roger
name: Chael, Andrew A.
name: Chamberlin, Richard
name: Chan, Chi-Kwan
name: Crew, Geoffrey B.
name: Dexter, Jason
name: Dexter, Matt
name: Dzib, Sergio A.
name: Falcke, Heino
name: Freund, Robert
name: Friberg, Per
name: Greer, Christopher H.
name: Gurwell, Mark A.
name: Ho, Paul T. P.
name: Honma, Mareki
name: Inoue, Makoto
name: Johannsen, Tim
name: Kim, Junhan
name: Krichbaum, Thomas P.
name: Lamb, James
name: León-Tavares, Jonathan
name: Loeb, Abraham
name: Loinard, Laurent
name: MacMahon, David
name: Marrone, Daniel P.
name: Moran, James M.
name: Mościbrodzka, Monika
name: Ortiz-León, Gisela N.
name: Oyama, Tomoaki
name: Özel, Feryal
name: Plambeck, Richard L.
name: Pradel, Nicolas
name: Primiani, Rurik A.
name: Rogers, Alan E. E.
name: Rosenfeld, Katherine
name: Rottmann, Helge
name: Roy, Alan L.
name: Ruszczyk, Chester
name: Smythe, Daniel L.
name: SooHoo, Jason
name: Spilker, Justin
name: Stone, Jordan
name: Strittmatter, Peter
name: Tilanus, Remo P. J.
name: Titus, Michael
name: Vertatschitsch, Laura
name: Wagner, Jan
name: Wardle, John F. C.
name: Weintroub, Jonathan
name: Woody, David
name: Wright, Melvyn
name: Yamaguchi, Paul
name: Young, André
name: Young, Ken H.
name: Zensus, J. Anton
name: Ziurys, Lucy M.
Publisher: name: arXiv
Year: 2016
Subjects: subject: Astrophysics of Galaxies (astro-ph.GA)
subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: Instrumentation and Methods for Astrophysics (astro-ph.IM)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: The Galactic Center black hole Sagittarius A* (Sgr A*) is a prime observing target for the Event Horizon Telescope (EHT), which can resolve the 1.3 mm emission from this source on angular scales comparable to that of the general relativistic shadow. Previous EHT observations have used visibility amplitudes to infer the morphology of the millimeter-wavelength emission. Potentially much richer source information is contained in the phases. We report on 1.3 mm phase information on Sgr A* obtained with the EHT on a total of 13 observing nights over 4 years. Closure phases, the sum of visibility phases along a closed triangle of interferometer baselines, are used because they are robust against phase corruptions introduced by instrumentation and the rapidly variable atmosphere. The median closure phase on a triangle including telescopes in California, Hawaii, and Arizona is nonzero. This result conclusively demonstrates that the millimeter emission is asymmetric on scales of a few Schwarzschild radii and can be used to break 180-degree rotational ambiguities inherent from amplitude data alone. The stability of the sign of the closure phase over most observing nights indicates persistent asymmetry in the image of Sgr A* that is not obscured by refraction due to interstellar electrons along the line of sight.
description: 11 pages, accepted to ApJ