Persistent Asymmetric Structure of Sagittarius A* on Event Horizon Scales

Citation:

Vincent L. Fish, Michael D. Johnson, Sheperd S. Doeleman, Avery E. Broderick, Dimitrios Psaltis, Ru-Sen Lu, Kazunori Akiyama, Walter Alef, Juan Carlos Algaba, Keiichi Asada, Christopher Beaudoin, Alessandra Bertarini, Lindy Blackburn, Ray Blundell, Geoffrey C. Bower, Christiaan Brinkerink, Roger Cappallo, Andrew A. Chael, Richard Chamberlin, Chi-Kwan Chan, Geoffrey B. Crew, Jason Dexter, Matt Dexter, Sergio A. Dzib, Heino Falcke, Robert Freund, Per Friberg, Christopher H. Greer, Mark A. Gurwell, Paul T. P. Ho, Mareki Honma, Makoto Inoue, Tim Johannsen, Junhan Kim, Thomas P. Krichbaum, James Lamb, Jonathan León-Tavares, Abraham Loeb, Laurent Loinard, David MacMahon, Daniel P. Marrone, James M. Moran, Monika Mościbrodzka, Gisela N. Ortiz-León, Tomoaki Oyama, Feryal Özel, Richard L. Plambeck, Nicolas Pradel, Rurik A. Primiani, Alan E. E. Rogers, Katherine Rosenfeld, Helge Rottmann, Alan L. Roy, Chester Ruszczyk, Daniel L. Smythe, Jason SooHoo, Justin Spilker, Jordan Stone, Peter Strittmatter, Remo P. J. Tilanus, Michael Titus, Laura Vertatschitsch, Jan Wagner, John F. C. Wardle, Jonathan Weintroub, David Woody, Melvyn Wright, Paul Yamaguchi, André Young, Ken H. Young, J. Anton Zensus, and Lucy M. Ziurys. 2016. “Persistent Asymmetric Structure of Sagittarius A* on Event Horizon Scales.” The Astrophysical Journal, 820.

Abstract:

The Galactic Center black hole Sagittarius A* (Sgr A*) is a primeobserving target for the Event Horizon Telescope (EHT), which canresolve the 1.3 mm emission from this source on angular scalescomparable to that of the general relativistic shadow. Previous EHTobservations have used visibility amplitudes to infer the morphology ofthe millimeter-wavelength emission. Potentially much richer sourceinformation is contained in the phases. We report on 1.3 mm phaseinformation on Sgr A* obtained with the EHT on a total of 13 observingnights over four years. Closure phases, which are the sum of visibilityphases along a closed triangle of interferometer baselines, are usedbecause they are robust against phase corruptions introduced byinstrumentation and the rapidly variable atmosphere. The median closurephase on a triangle including telescopes in California, Hawaii, andArizona is nonzero. This result conclusively demonstrates that themillimeter emission is asymmetric on scales of a few Schwarzschild radiiand can be used to break 180° rotational ambiguities inherent fromamplitude data alone. The stability of the sign of the closure phaseover most observing nights indicates persistent asymmetry in the imageof Sgr A* that is not obscured by refraction due to interstellarelectrons along the line of sight.