Detection of Intrinsic Source Structure at ∼3 Schwarzschild Radii with Millimeter-VLBI Observations of SAGITTARIUS A∗

Ru Sen Lu, Thomas P. Krichbaum, Alan L. Roy, Vincent L. Fish, Sheperd S. Doeleman, Michael D. Johnson, Kazunori Akiyama, Dimitrios Psaltis, Walter Alef, Keiichi Asada, Christopher Beaudoin, Alessandra Bertarini, Lindy Blackburn, Ray Blundell, Geoffrey C. Bower, Christiaan Brinkerink, Avery E. Broderick, Roger Cappallo, Geoffrey B. Crew, Jason DexterMatt Dexter, Heino Falcke, Robert Freund, Per Friberg, Christopher H. Greer, Mark A. Gurwell, Paul T.P. Ho, Mareki Honma, Makoto Inoue, Junhan Kim, James Lamb, Michael Lindqvist, David Macmahon, Daniel P. Marrone, Ivan Marti-Vidal, Karl M. Menten, James M. Moran, Neil M. Nagar, Richard L. Plambeck, Rurik A. Primiani, Alan E.E. Rogers, Eduardo Ros, Helge Rottmann, Jason Soohoo, Justin Spilker, Jordan Stone, Peter Strittmatter, Remo P.J. Tilanus, Michael Titus, Laura Vertatschitsch, Jan Wagner, Jonathan Weintroub, Melvyn Wright, Ken H. Young, J. Anton Zensus, Lucy M. Ziurys

Research output: Contribution to journalArticle

25 Scopus citations


We report results from very long baseline interferometric (VLBI) observations of the supermassive black hole in the Galactic center, Sgr A∗, at 1.3 mm (230 GHz). The observations were performed in 2013 March using six VLBI stations in Hawaii, California, Arizona, and Chile. Compared to earlier observations, the addition of the APEX telescope in Chile almost doubles the longest baseline length in the array, provides additional uv coverage in the N-S direction, and leads to a spatial resolution of ∼30 μas (∼3 Schwarzschild radii) for Sgr A∗. The source is detected even at the longest baselines with visibility amplitudes of ∼4%-13% of the total flux density. We argue that such flux densities cannot result from interstellar refractive scattering alone, but indicate the presence of compact intrinsic source structure on scales of ∼3 Schwarzschild radii. The measured nonzero closure phases rule out point-symmetric emission. We discuss our results in the context of simple geometric models that capture the basic characteristics and brightness distributions of disk- and jet-dominated models and show that both can reproduce the observed data. Common to these models are the brightness asymmetry, the orientation, and characteristic sizes, which are comparable to the expected size of the black hole shadow. Future 1.3 mm VLBI observations with an expanded array and better sensitivity will allow more detailed imaging of the horizon-scale structure and bear the potential for a deep insight into the physical processes at the black hole boundary.

Original languageEnglish (US)
Article number60
JournalAstrophysical Journal
Issue number1
StatePublished - May 20 2018


  • Galaxy: center
  • submillimeter: general
  • techniques: high angular resolution
  • techniques: interferometric

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Lu, R. S., Krichbaum, T. P., Roy, A. L., Fish, V. L., Doeleman, S. S., Johnson, M. D., Akiyama, K., Psaltis, D., Alef, W., Asada, K., Beaudoin, C., Bertarini, A., Blackburn, L., Blundell, R., Bower, G. C., Brinkerink, C., Broderick, A. E., Cappallo, R., Crew, G. B., ... Ziurys, L. M. (2018). Detection of Intrinsic Source Structure at ∼3 Schwarzschild Radii with Millimeter-VLBI Observations of SAGITTARIUS A∗. Astrophysical Journal, 859(1), [60].