title: Resolved magnetic-field structure and variability near the event horizon of Sagittarius A.
abstract: Near a black hole, differential rotation of a magnetized accretion disk is thought to produce an instability that amplifies weak magnetic fields, driving accretion and outflow. These magnetic fields would naturally give rise to the observed synchrotron emission in galaxy cores and to the formation of relativistic jets, but no observations to date have been able to resolve the expected horizon-scale magnetic-field structure. We report interferometric observations at 1.3-millimeter wavelength that spatially resolve the linearly polarized emission from the Galactic Center supermassive black hole, Sagittarius A*. We have found evidence for partially ordered magnetic fields near the event horizon, on scales of ~6 Schwarzschild radii, and we have detected and localized the intrahour variability associated with these fields.
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA. mjohnson@cfa.harvard.edu.
authors.forename: Michael D
authors.initials: MD
authors.lastname: Johnson
authors.affiliation: Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA.
authors.forename: Vincent L
authors.initials: VL
authors.lastname: Fish
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA. Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA.
authors.forename: Sheperd S
authors.initials: SS
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authors.affiliation: Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA.
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Marrone
authors.affiliation: Department of Astronomy, Radio Astronomy Laboratory, 501 Campbell, University of California Berkeley, Berkeley, CA 94720-3411, USA.
authors.forename: Richard L
authors.initials: RL
authors.lastname: Plambeck
authors.affiliation: Department of Physics MS-057, Brandeis University, Waltham, MA 02454-0911.
authors.forename: John F C
authors.initials: JF
authors.lastname: Wardle
authors.affiliation: Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA. National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan. Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors.forename: Kazunori
authors.initials: K
authors.lastname: Akiyama
authors.affiliation: Institute of Astronomy and Astrophysics, Academia Sinica, Post Office Box 23-141, Taipei 10617, Taiwan.
authors.forename: Keiichi
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authors.affiliation: Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA.
authors.forename: Christopher
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authors.lastname: Beaudoin
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
authors.forename: Lindy
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authors.lastname: Blackburn
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
authors.forename: Ray
authors.initials: R
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authors.affiliation: Academia Sinica Institute for Astronomy and Astrophysics (ASIAA), 645 N. A'ohōkū Pl. Hilo, HI 96720, USA.
authors.forename: Geoffrey C
authors.initials: GC
authors.lastname: Bower
authors.affiliation: Department of Astrophysics/Institute for Mathematics, Astrophysics and Particle Physics, Radboud University Nijmegen, Post Office Box 9010, 6500 GL Nijmegen, Netherlands.
authors.forename: Christiaan
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authors.lastname: Brinkerink
authors.affiliation: Perimeter Institute for Theoretical Physics, 31 Caroline Street North, Waterloo, ON N2L 2Y5, Canada. Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada.
authors.forename: Avery E
authors.initials: AE
authors.lastname: Broderick
authors.affiliation: Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA.
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authors.lastname: Cappallo
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Chael
authors.affiliation: Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA.
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authors.affiliation: Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany.
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authors.affiliation: Department of Astronomy, Radio Astronomy Laboratory, 501 Campbell, University of California Berkeley, Berkeley, CA 94720-3411, USA.
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authors.lastname: Dexter
authors.affiliation: Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA.
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authors.affiliation: James Clerk Maxwell Telescope, East Asia Observatory, 660 N. A'ohōkū Place, University Park, Hilo, HI 96720, USA.
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authors.lastname: Friberg
authors.affiliation: Department of Physics, Joint Space-Science Institute, University of Maryland at College Park, Physical Sciences Complex, College Park, MD 20742, USA.
authors.forename: Roman
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authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
authors.forename: Mark A
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authors.affiliation: Institute of Astronomy and Astrophysics, Academia Sinica, Post Office Box 23-141, Taipei 10617, Taiwan.
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authors.initials: PT
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authors.affiliation: National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan. Graduate University for Advanced Studies, Mitaka, 2-21-1 Osawa, Mitaka, Tokyo 181-8588.
authors.forename: Mareki
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authors.affiliation: Institute of Astronomy and Astrophysics, Academia Sinica, Post Office Box 23-141, Taipei 10617, Taiwan.
authors.forename: Makoto
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authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA. Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA. Department of Physics MS-057, Brandeis University, Waltham, MA 02454-0911.
authors.forename: Michael
authors.initials: M
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authors.affiliation: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.
authors.forename: Thomas P
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authors.forename: James
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authors.lastname: Lamb
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
authors.forename: Abraham
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authors.affiliation: Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA. Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.
authors.forename: Ru-Sen
authors.initials: RS
authors.lastname: Lu
authors.affiliation: Department of Astronomy, Radio Astronomy Laboratory, 501 Campbell, University of California Berkeley, Berkeley, CA 94720-3411, USA.
authors.forename: David
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authors.lastname: MacMahon
authors.affiliation: Department of Physics, Joint Space-Science Institute, University of Maryland at College Park, Physical Sciences Complex, College Park, MD 20742, USA.
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authors.initials: JC
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authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
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authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
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authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
authors.forename: Rurik A
authors.initials: RA
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authors.affiliation: Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA.
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authors.affiliation: Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA.
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authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
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authors.affiliation: Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA.
authors.forename: Jason
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authors.lastname: SooHoo
authors.affiliation: Department of Astrophysics/Institute for Mathematics, Astrophysics and Particle Physics, Radboud University Nijmegen, Post Office Box 9010, 6500 GL Nijmegen, Netherlands. Leiden Observatory, Leiden University, Post Office Box 9513, 2300 RA Leiden, Netherlands.
authors.forename: Remo P J
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authors.affiliation: Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA.
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authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
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authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
authors.forename: Jonathan
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authors.affiliation: Department of Astronomy, Radio Astronomy Laboratory, 501 Campbell, University of California Berkeley, Berkeley, CA 94720-3411, USA.
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authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
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authors.affiliation: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.
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authors.affiliation: Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA.
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