Robert Freund, N. W. Halverson, Benjamin R. Ryan, Paul Shaw, André Young, Andreas Eckart, A. Montaña, Michael Titus, Chen Yu Yu, Gertie Geertsema, Gao Feng, Ronald Grosslein, Ranjani Srinivasan, David Ball, Pablo Torne, Roberto Garcia, Hiroki Okino, Kotaro Moriyama, Chris Eckert, Lupin C.C. Lin, Geoffrey B. Crew, Vernon Fath, Freek Roelofs, David Sánchez-Arguelles, Luciano Rezzolla, Makoto Inoue, Michael Bremer, Jongsoo Kim, James Hoge, Michael Janssen, David M. Gale, Mel Rose, Jason Dexter, Do-Young Byun, J. G. A. Wouterloot, Rubén Herrero-Illana, Daniel C. M. Palumbo, Ta Shun Wei, Ching Tang Liu, Bradford Benson, Jadyn Anczarski, Patrick M. Koch, Ken Young, Jae-Young Kim, Minfeng Gu, Mislav Baloković, Mariafelicia De Laurentis, Laurent Loinard, A. Jiménez-Rosales, Tomohisa Kawashima, Nicolas Pradel, Heino Falcke, Olivier Gentaz, Dirk Muders, Shami Chatterjee, Britton Jeter, Rocco Lico, Craig Walther, David J. James, Homin Jiang, Michael H. Hecht, Gopal Narayanan, Qingwen Wu, Pierre Martin-Cocher, Michael A. Nowak, Alexander W. Raymond, Gregory Desvignes, Anne Kathrin Baczko, Chet Ruszczyk, Yutaka Hasegawa, Chao-Te Li, M. C. H. Wright, Andrew Nadolski, Alan P. Marscher, Christopher Beaudoin, Harriet Parsons, Peñalver Juan, Karl M. Menten, Silke Britzen, Frédéric Gueth, Shu Hao Chang, Andrew Chael, Daryl Haggard, Rodrigo Córdova Rosado, Ru-Sen Lu, Mansour Karami, José L. Gómez, Sang-Sung Lee, Tirupati K. Sridharan, Karl Friedrich Schuster, Ronald Hesper, Richard L. Plambeck, Iain Coulson, Jason SooHoo, Aristeidis Noutsos, Svetlana G. Jorstad, Li Ming Lu, James M. Cordes, David H. Hughes, Jonathan Weintroub, Chih-Wei Locutus Huang, Katherine L. Bouman, Roger J. Cappallo, Lijing Shao, Christiaan D. Brinkerink, John Kuroda, Ramesh Karuppusamy, Iniyan Natarajan, Jessica Dempsey, George Nystrom, John E. Carlstrom, Sera Markoff, Mark Kettenis, Neal R. Erickson, Jason W. Henning, R. Laing, Huang Lei, Kevin A. Dudevoir, Ilje Cho, William Stahm, Juan-Carlos Algaba, Junhan Kim, Hotaka Shiokawa, Martin P. McColl, James M. Moran, Chi-kwan Chan, Timothy C. Chuter, Thomas W. Folkers, Yi Chen, Christopher Greer, Lia Medeiros, C. Y. Kuo, Kuo Chang Han, Shoko Koyama, William Snow, Rurik A. Primiani, Sjoerd T. Timmer, F. Peter Schloerb, Stephen R. McWhirter, Fumie Tazaki, Norbert Wex, Ming-Tang Chen, Nimesh A. Patel, Aaron Faber, Mark Derome, Kazunori Akiyama, W. B. Everett, Hiroshi Nagai, Andrei Lobanov, Ignacio Ruiz, Pierre Christian, N. Phillips, David C. Forbes, Don Sousa, Michael Lindqvist, Christopher Risacher, Wen Ping Lo, Geoffrey C. Bower, Bart Ripperda, Dominique Broguiere, Maciek Wielgus, Antony A. Stark, Raquel Fraga-Encinas, Hiroaki Nishioka, Philippe Raffin, Hugo Messias, Feryal Özel, Jun Yi Koay, Buell T. Jannuzi, Sandra Bustamente, Roberto Neri, Jinchi Hao, Ye-Fei Yuan, Garrett K. Keating, Lynn D. Matthews, Avery E. Broderick, Mark G. Rawlings, Ciriaco Goddi, Tod R. Lauer, Kamal Souccar, Alan L. Roy, S. Navarro, Luis C. Ho, Timothy Norton, Roger Brissenden, Doosoo Yoon, Jongho Park, Richard Lacasse, Paul T. P. Ho, Derek Ward-Thompson, Atish Kamble, Akihiko Hirota, S. Sánchez, D. A. Graham, Vincent Piétu, Kyle D. Massingill, M. Zeballos, Mahito Sasada, Hideo Ogawa, Ziri Younsi, Chih Cheng Chang, Alejandro F. Sáez-Madain, Christian M. Fromm, Ramesh Narayan, Shuichiro Tsuda, Ryan Berthold, Gibwa Musoke, Laura Vertatschitsch, Masanori Nakamura, Remo P. J. Tilanus, Cornelia Müller, Kimihiro Kimura, Roman Gold, Venkatessh Ramakrishnan, Yuzhu Cui, Frederick K. Baganoff, Alan R. Whitney, Aleksandar Popstefanija, Helge Rottmann, Yuan Feng, Ralph Eatough, Tuomas Savolainen, Neil M. Nagar, Alexander Allardi, M. Mora-Klein, Thomas Bronzwaer, Mark Gurwell, Bong Won Sohn, Ivan Marti-Vidal, Chih Chiang Han, Hung Yi Pu, Yan-Rong Li, Shan Shan Zhao, Song Chu Chang, Zhi-Qiang Shen, John F. C. Wardle, Carsten Kramer, Koushik Chatterjee, Wagner Jan, Tomoaki Oyama, Ray Blundell, Motoki Kino, Alan E. E. Rogers, Rebecca Azulay, Jordy Davelaar, Tyler Trent, Satoki Matsushita, Kazi L.J. Rygl, Shuo Zhang, John E. Barrett, Peter Oshiro, Ryan Chilson, Jorge A. Preciado-López, Daniel Michalik, Peter Galison, Uwe Bach, Ilse van Bemmel, Pim Schellart, Michael D. Johnson, Jiang Wu, J. Anton Zensus, S. A. Dzib, Arturo I. Gómez-Ruiz, Meyer Zhao Zheng, David John, Dimitrios Psaltis, Daniel P. Marrone, M. Poirier, Shiro Ikeda, Ralph G. Marson, A. Hernandez-Gomez, Sven Dornbusch, George Reiland, Mareki Honma, J. Blanchard, Ed Fomalont, Taehyun Jung, Izumi Mizuno, Monika Moscibrodzka, Vincent L. Fish, Matthew R. Dexter, Paul Tiede, Rodrigo Amestica, Nicholas R. MacDonald, Gisela N. Ortiz-León, Colin J. Lonsdale, Callie Matulonis, Charles F. Gammie, Per Friberg, Boris Georgiev, W. Boland, Ramprasad Rao, Guang-Yao Zhao, Joseph R. Farah, Zhiyuan Li, Hector Olivares, Sara Issaoun, Elisabetta Liuzzo, C. M. Violette Impellizzeri, Michael Kramer, Oliver Porth, Thomas P. Krichbaum, Dominic W. Pesce, Daniel R. van Rossum, David R. Smith, Robert Wharton, Kuan Yu Liu, David P. Woody, Arash Roshanineshat, Sheperd S. Doeleman, Chung Chen Chen, Ziyan Zhu, Ue-Li Pen, Yosuke Mizuno, Prather Ben, Sascha Trippe, Walter Alef, Liu Kuo, Alexandra S. Rahlin, William Montgomerie, George N. Wong, Jirong Mao, Kazuhiro Hada, Efthalia Traianou, John Conway, Remi Sassella, Eduardo Ros, Kevin M. Silva, Derek Kubo, E. Castillo-Domínguez, Huib Jan van Langevelde, Keiichi Asada, Des Small, Joseph Neilsen, Chi H. Nguyen, Chunchong Ni, Yusuke Kono, Ryan Keisler, Paul Yamaguchi, Lindy Blackburn, Erik M. Leitch, Roger Deane, Lucy M. Ziurys, K. T. Story, Joseph Crowley, Nathan Whitehorn, Stefan Heyminck, Kenji Toma, Antxon Alberdi, Yau De Huang, Dan Bintley, Y Kim, J., Krichbaum, T. P., Broderick, A. E., Wielgus, M., Blackburn, L., Gomez, J. -L., Johnson, M. D., Bouman, K. L., Chael, A., Akiyama, K., Jorstad, S., Marscher, A. P., Issaoun, S., Janssen, M., Chan, C. -K., Savolainen, T., Pesce, D. W., Ozel, F., De Laurentis, M., Deane, R., Institut de RadioAstronomie Millimétrique (IRAM), Centre National de la Recherche Scientifique (CNRS), Laboratoire d'études spatiales et d'instrumentation en astrophysique (LESIA (UMR_8109)), Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université Paris Diderot - Paris 7 (UPD7)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), Instituto de RadioAstronomía Milimétrica (IRAM), Event Horizon Telescope, Ministerio de Economía y Competitividad (España), European Commission, National Aeronautics and Space Administration (US), Max Planck Institute for Radio Astronomy, Perimeter Institute for Theoretical Physics, Harvard University, CSIC, Princeton University, Boston University, Radboud University Nijmegen, University of Arizona, Anne Lähteenmäki Group, Academia Sinica - Institute of Astronomy and Astrophysics, Massachusetts Institute of Technology, East Asian Observatory, Nederlandse Onderzoekschool voor Astronomie, Academia Sinica, Institut de Radio Astronomie Millimétrique, Korea Astronomy and Space Science Institute, University of Chicago, Cornell University, University of Amsterdam, CAS - Shanghai Astronomical Observatory, Chalmers University of Technology, National Astronomical Observatory of Japan, University of Naples Federico II, University of Pretoria, University of Colorado Boulder, National Radio Astronomy Observatory, Goethe University Frankfurt, University of Illinois at Urbana-Champaign, University of Waterloo, Instituto Nacional de Astrofisica Optica y Electronica, University of Groningen, Peking University, Max Planck Institute for Extraterrestrial Physics, Joint Institute for VLBI in Europe, California Institute of Technology, National Sun Yat-sen University, National Optical Astronomy Observatory, CAS - Institute of High Energy Physics, Nanjing University, INAF Istituto di Radioastronomia, Instituto de Radioastronomía y Astrofísica, CAS - National Astronomical Observatories, Universidad de Valencia, Universidad de Concepción, University of Massachusetts, Rhodes University, University of California Berkeley, Los Alamos National Laboratory, IRAM, Tohoku University, Seoul National University, Brandeis University, University of Central Lancashire, Huazhong University of Science and Technology, University of Science and Technology of China, University of Vermont, Villanova University, United States Department of Energy, Western University, Royal Netherlands Meteorological Institute, McGill University, Osaka Prefecture University, European Southern Observatory Santiago, University of Manchester, National Radio Astronomy Observatory Socorro, Rochester Institute of Technology, Washington University St. Louis, Systems and Technology Research, Georgia Institute of Technology, Stanford University, University of California Los Angeles, Department of Electronics and Nanoengineering, Aalto-yliopisto, Aalto University, Astronomy, High Energy Astrophys. & Astropart. Phys (API, FNWI), and Gravitation and Astroparticle Physics Amsterdam
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Open Access funding provided by Max Planck Society.--All authors: Kim, Jae-Young; Krichbaum, Thomas P.; Broderick, Avery E.; Wielgus, Maciek; Blackburn, Lindy; Gómez, José L.; Johnson, Michael D.; Bouman, Katherine L.; Chael, Andrew; Akiyama, Kazunori; Jorstad, Svetlana; Marscher, Alan P.; Issaoun, Sara; Janssen, Michael; Chan, Chi-kwan; Savolainen, Tuomas; Pesce, Dominic W.; Özel, Feryal; Alberdi, Antxon; Alef, Walter Asada, Keiichi; Azulay, Rebecca; Baczko, Anne-Kathrin; Ball, David; Baloković, Mislav; Barrett, John; Bintley, Dan; Boland, Wilfred; Bower, Geoffrey C.; Bremer, Michael; Brinkerink, Christiaan D.; Brissenden, Roger; Britzen, Silke; Broguiere, Dominique; Bronzwaer, Thomas; Byun, Do-Young; Carlstrom, John E.; Chatterjee, Shami; Chatterjee, Koushik; Chen, Ming-Tang; Chen, Yongjun; Cho, Ilje; Christian, Pierre; Conway, John E.; Cordes, James M.; Crew, Geoffrey B.; Cui, Yuzhu; Davelaar, Jordy; De Laurentis, Mariafelicia; Deane, Roger; Dempsey, Jessica; Desvignes, Gregory; Dexter, Jason; Doeleman, Sheperd S.; Eatough, Ralph P.; Falcke, Heino; Fish, Vincent L.; Fomalont, Ed; Fraga-Encinas, Raquel; Friberg, Per; Fromm, Christian M.; Galison, Peter; Gammie, Charles F.; García, Roberto; Gentaz, Olivier; Georgiev, Boris; Goddi, Ciriaco; Gold, Roman; Gómez-Ruiz, Arturo I.; Gu, Minfeng; Gurwell, Mark; Hada, Kazuhiro; Hecht, Michael H.; Hesper, Ronald; Ho, Luis C.; Ho, Paul; Honma, Mareki; Huang, Chih-Wei L.; Huang, Lei; Hughes, David H.; Ikeda, Shiro; Inoue, Makoto; James, David J.; Jannuzi, Buell T.; Jeter, Britton; Jiang, Wu; Jimenez-Rosales, Alejandra; Jung, Taehyun; Karami, Mansour; Karuppusamy, Ramesh; Kawashima, Tomohisa; Keating, Garrett K.; Kettenis, Mark; Kim, Junhan; Kim, Jongsoo; Kino, Motoki; Koay, Jun Yi; Koch, Patrick M.; Koyama, Shoko; Kramer, Michael; Kramer, Carsten; Kuo, Cheng-Yu; Lauer, Tod R.; Lee, Sang-Sung; Li, Yan-Rong; Li, Zhiyuan; Lindqvist, Michael; Lico, Rocco; Liu, Kuo; Liuzzo, Elisabetta; Lo, Wen-Ping; Lobanov, Andrei P.; Loinard, Laurent; Lonsdale, Colin; Lu, Ru-Sen; MacDonald, Nicholas R.; Mao, Jirong; Markoff, Sera; Marrone, Daniel P.; Martí-Vidal, Iván; Matsushita, Satoki; Matthews, Lynn D.; Medeiros, Lia; Menten, Karl M.; Mizuno, Yosuke; Mizuno, Izumi; Moran, James M.; Moriyama, Kotaro; Moscibrodzka, Monika; Musoke, Gibwa; Müller, Cornelia; Nagai, Hiroshi; Nagar, Neil M.; Nakamura, Masanori; Narayan, Ramesh; Narayanan, Gopal; Natarajan, Iniyan; Neri, Roberto; Ni, Chunchong; Noutsos, Aristeidis; Okino, Hiroki; Olivares, Héctor; Ortiz-León, Gisela N.; Oyama, Tomoaki; Palumbo, Daniel C. M.; Park, Jongho; Patel, Nimesh; Pen, Ue-Li; Piétu, Vincent; Plambeck, Richard; PopStefanija, Aleksandar; Porth, Oliver; Prather, Ben; Preciado-López, Jorge A.; Psaltis, Dimitrios; Pu, Hung-Yi; Ramakrishnan, Venkatessh; Rao, Ramprasad; Rawlings, Mark G.; Raymond, Alexander W.; Rezzolla, Luciano; Ripperda, Bart; Roelofs, Freek; Rogers, Alan; Ros, Eduardo; Rose, Mel; Roshanineshat, Arash; Rottmann, Helge; Roy, Alan L.; Ruszczyk, Chet; Ryan, Benjamin R.; Rygl, Kazi L. J.; Sánchez, Salvador; Sánchez-Arguelles, David; Sasada, Mahito; Schloerb, F. Peter; Schuster, Karl-Friedrich; Shao, Lijing; Shen, Zhiqiang; Small, Des; Sohn, Bong Won; SooHoo, Jason; Tazaki, Fumie; Tiede, Paul; Tilanus, Remo P. J.; Titus, Michael; Toma, Kenji; Torne, Pablo; Trent, Tyler; Traianou, Efthalia; Trippe, Sascha; Tsuda, Shuichiro; van Bemmel, Ilse; van Langevelde, Huib Jan; van Rossum, Daniel R.; Wagner, Jan; Wardle, John; Ward-Thompson, Derek; Weintroub, Jonathan; Wex, Norbert; Wharton, Robert; Wong, George N.; Wu, Qingwen; Yoon, Doosoo; Young, André; Young, Ken; Younsi, Ziri; Yuan, Feng; Yuan, Ye-Fei; Zensus, J. Anton; Zhao, Guangyao; Zhao, Shan-Shan; Zhu, Ziyan; Algaba, Juan-Carlos; Allardi, Alexander; Amestica, Rodrigo; Anczarski, Jadyn; Bach, Uwe; Baganoff, Frederick K.; Beaudoin, Christopher; Benson, Bradford A.; Berthold, Ryan; Blanchard, Jay M.; Blundell, Ray; Bustamente, Sandra; Cappallo, Roger; Castillo-Domínguez, Edgar; Chang, Chih-Cheng; Chang, Shu-Hao; Chang, Song-Chu; Chen, Chung-Chen; Chilson, Ryan; Chuter, Tim C.; Rosado, Rodrigo Córdova; Coulson, Iain M.; Crowley, Joseph; Derome, Mark; Dexter, Matthew; Dornbusch, Sven; Dudevoir, Kevin A.; Dzib, Sergio A.; Eckart, Andreas; Eckert, Chris; Erickson, Neal R.; Everett, Wendeline B.; Faber, Aaron; Farah, Joseph R.; Fath, Vernon; Folkers, Thomas W.; Forbes, David C.; Freund, Robert; Gale, David M.; Gao, Feng; Geertsema, Gertie; Graham, David A.; Greer, Christopher H.; Grosslein, Ronald; Gueth, Frédéric; Haggard, Daryl; Halverson, Nils W.; Han, Chih-Chiang; Han, Kuo-Chang; Hao, Jinchi; Hasegawa, Yutaka; Henning, Jason W.; Hernández-Gómez, Antonio; Herrero-Illana, Rubén; Heyminck, Stefan; Hirota, Akihiko; Hoge, James; Huang, Yau-De; Violette Impellizzeri, C. M.; Jiang, Homin; John, David; Kamble, Atish; Keisler, Ryan; Kimura, Kimihiro; Kono, Yusuke; Kubo, Derek; Kuroda, John; Lacasse, Richard; Laing, Robert A.; Leitch, Erik M.; Li, Chao-Te; Lin, Lupin C. -C.; Liu, Ching-Tang; Liu, Kuan-Yu; Lu, Li-Ming; Marson, Ralph G.; Martin-Cocher, Pierre L.; Massingill, Kyle D.; Matulonis, Callie; McColl, Martin P.; McWhirter, Stephen R.; Messias, Hugo; Meyer-Zhao, Zheng; Michalik, Daniel; Montaña, Alfredo; Montgomerie, William; Mora-Klein, Matias; Muders, Dirk; Nadolski, Andrew; Navarro, Santiago; Neilsen, Joseph; Nguyen, Chi H.; Nishioka, Hiroaki; Norton, Timothy; Nowak, Michael A.; Nystrom, George; Ogawa, Hideo; Oshiro, Peter; Oyama, Tomoaki; Parsons, Harriet; Peñalver, Juan; Phillips, Neil M.; Poirier, Michael; Pradel, Nicolas; Primiani, Rurik A.; Raffin, Philippe A.; Rahlin, Alexandra S.; Reiland, George; Risacher, Christopher; Ruiz, Ignacio; Sáez-Madaín, Alejandro F.; Sassella, Remi; Schellart, Pim; Shaw, Paul; Silva, Kevin M.; Shiokawa, Hotaka; Smith, David R.; Snow, William; Souccar, Kamal; Sousa, Don; Sridharan, Tirupati K.; Srinivasan, Ranjani; Stahm, William; Stark, Antony A.; Story, Kyle; Timmer, Sjoerd T.; Vertatschitsch, Laura; Walther, Craig; Wei, Ta-Shun; Whitehorn, Nathan; Whitney, Alan R.; Woody, David P.; Wouterloot, Jan G. A.; Wright, Melvin; Yamaguchi, Paul; Yu, Chen-Yu; Zeballos, Milagros; Zhang, Shuo; Ziurys, Lucy; Event Horizon Telescope Collaboration, 3C 279 is an archetypal blazar with a prominent radio jet that show broadband flux density variability across the entire electromagnetic spectrum. We use an ultra-high angular resolution technique - global Very Long Baseline Interferometry (VLBI) at 1.3mm (230 GHz) - to resolve the innermost jet of 3C 279 in order to study its fine-scale morphology close to the jet base where highly variable-ray emission is thought to originate, according to various models. The source was observed during four days in April 2017 with the Event Horizon Telescope at 230 GHz, including the phased Atacama Large Millimeter/submillimeter Array, at an angular resolution of ∼20 μas (at a redshift of z = 0:536 this corresponds to ∼0:13 pc ∼ 1700 Schwarzschild radii with a black hole mass MBH = 8 × 108 M⊙). Imaging and model-fitting techniques were applied to the data to parameterize the fine-scale source structure and its variation.We find a multicomponent inner jet morphology with the northernmost component elongated perpendicular to the direction of the jet, as imaged at longer wavelengths. The elongated nuclear structure is consistent on all four observing days and across diffierent imaging methods and model-fitting techniques, and therefore appears robust. Owing to its compactness and brightness, we associate the northern nuclear structure as the VLBI "core". This morphology can be interpreted as either a broad resolved jet base or a spatially bent jet.We also find significant day-to-day variations in the closure phases, which appear most pronounced on the triangles with the longest baselines. Our analysis shows that this variation is related to a systematic change of the source structure. Two inner jet components move non-radially at apparent speeds of ∼15 c and ∼20 c (∼1:3 and ∼1:7 μas day-1, respectively), which more strongly supports the scenario of traveling shocks or instabilities in a bent, possibly rotating jet. The observed apparent speeds are also coincident with the 3C 279 large-scale jet kinematics observed at longer (cm) wavelengths, suggesting no significant jet acceleration between the 1.3mm core and the outer jet. The intrinsic brightness temperature of the jet components are ≤1010 K, a magnitude or more lower than typical values seen at ≥7mm wavelengths. The low brightness temperature and morphological complexity suggest that the core region of 3C 279 becomes optically thin at short (mm) wavelengths. © J.-Y. Kim et al. 2020., The authors of the present paper thank the following organizations and programs: the Academy of Finland (projects 274477, 284495, 312496); the Advanced European Network of E-infrastructures for Astronomy with the SKA (AENEAS) project, supported by the European Commission Framework Programme Horizon 2020 Research and Innovation action under grant agreement 731016; the Alexander von Humboldt Stiftung; the Black Hole Initiative at Harvard University, through a grant (60477) from the John Templeton Foundation; the China Scholarship Council; Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT, Chile, via PIA ACT172033, Fondecyt projects 1171506 and 3190878, BASAL AFB-170002, ALMA-conicyt 31140007); Consejo Nacional de Ciencia y Tecnologia (CONACYT, Mexico, projects 104497, 275201, 279006, 281692); the Delaney Family via the Delaney Family John A. Wheeler Chair at Perimeter Institute; Direccion General de Asuntos del Personal Academico -Universidad Nacional Autonoma de Mexico (DGAPA -UNAM, project IN112417); the European Research Council Synergy Grant "BlackHoleCam: Imaging the Event Horizon of Black Holes" (grant 610058); the Generalitat Valenciana postdoctoral grant APOSTD/2018/177 and GenT Program (project CIDEGENT/2018/021); the Gordon and Betty Moore Foundation (grants GBMF-3561, GBMF-5278); the Istituto Nazionale di Fisica Nucleare (INFN) sezione di Napoli, iniziative specifiche TEONGRAV; the International Max Planck Research School for Astronomy and Astrophysics at the Universities of Bonn and Cologne; the Jansky Fellowship program of the National Radio Astronomy Observatory (NRAO); the Japanese Government (Monbukagakusho: MEXT) Scholarship; the Japan Society for the Promotion of Science (JSPS) Grant-inAid for JSPS Research Fellowship (JP17J08829); the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (CAS, grants QYZDJSSW-SLH057, QYZDJ-SSW-SYS008, ZDBS-LY-SLH011); the Leverhulme Trust Early Career Research Fellowship; the Malaysian Fundamental Research Grant Scheme (FRGS, grant FRGS/1/2019/STG02/UM/02/6); the Max-PlanckGesellschaft (MPG); the Max Planck Partner Group of the MPG and the CAS; the MEXT/JSPS KAKENHI (grants 18KK0090, JP18K13594, JP18K03656, JP18H03721, 18K03709, 18H01245, 25120007); the MIT International Science and Technology Initiatives (MISTI) Funds; the Ministry of Science and Technology (MOST) of Taiwan (105-2112-M-001-025-MY3, 106-2112-M-001-011, 106-2119-M-001-027, 107-2119-M-001-017, 107-2119-M-001-020, and 107-2119-M-110-005); the National Aeronautics and Space Administration (NASA, Fermi Guest Investigator grant 80NSSC17K0649 and Hubble Fellowship grant HST-HF2-51431.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. r , for NASA, under contract NAS5-26555); the National Institute of Natural Sciences (NINS) of Japan; the National Key Research and Development Program of China (grant 2016YFA0400704, 2016YFA0400702); the National Science Foundation (NSF, grants AST-0096454, AST-0352953, AST-0521233, AST-0705062, AST-0905844, AST-0922984, AST-1126433, AST-1140030, DGE-1144085, AST-1207704, AST-1207730, AST-1207752, MRI-1228509, OPP-1248097, AST-1310896, AST-1312651, AST-1337663, AST-1440254, AST-1555365, AST-1715061, AST-1615796, AST-1716327, OISE-1743747, AST-1816420); the Natural Science Foundation of China (grants 11573051, 11633006, 11650110427, 10625314, 11721303, 11725312, 11933007); the Natural Sciences and Engineering Research Council of Canada (NSERC, including a Discovery Grant and the NSERC Alexander Graham Bell Canada Graduate Scholarships-Doctoral Program); the National Youth Thousand Talents Program of China; the National Research Foundation of Korea (the Global PhD Fellowship Grant: grants NRF-2015H1A2A1033752, 2015-R1D1A1A01056807, the Korea Research Fellowship Program: NRF-2015H1D3A1066561); the Netherlands Organization for Scientific Research (NWO) VICI award (grant 639.043.513) and Spinoza Prize SPI 78-409; the New Scientific Frontiers with Precision Radio Interferometry Fellowship awarded by the South African Radio Astronomy Observatory (SARAO), which is a facility of the National Research Foundation (NRF), an agency of the Department of Science and Technology (DST) of South Africa; the Onsala Space Observatory (OSO) national infrastructure, for the provisioning of its facilities/observational support (OSO receives funding through the Swedish Research Council under grant 2017-00648) the Perimeter Institute for Theoretical Physics (research at Perimeter Institute is supported by the Government of Canada through the Department of Innovation, Science and Economic Development and by the Province of Ontario through the Ministry of Research, Innovation and Science); the Russian Science Foundation (grant 17-12-01029); the Spanish Ministerio de Economia y Competitividad (grants PGC2018-098915-B-C21, AYA201680889-P); the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709); the Toray Science Foundation; the US Department of Energy (USDOE) through the Los Alamos National Laboratory (operated by Triad National Security, LLC, for the National Nuclear Security Administration of the USDOE (Contract 89233218CNA000001)); the Italian Ministero dell'Istruzione Universita e Ricerca through the grant Progetti Premiali 2012-iALMA (CUP C52I13000140001); the European Union's Horizon 2020 research and innovation programme under grant agreement No 730562 RadioNet; ALMA North America Development Fund; the Academia Sinica; Chandra TM6-17006X; the GenT Program (Generalitat Valenciana) Project CIDEGENT/2018/021. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), supported by NSF grant ACI-1548562, and CyVerse, supported by NSF grants DBI-0735191, DBI-1265383, and DBI1743442. XSEDE Stampede2 resource at TACC was allocated through TGAST170024 and TG-AST080026N. XSEDE JetStream resource at PTI and TACC was allocated through AST170028. The simulations were performed in part on the SuperMUC cluster at the LRZ in Garching, on the LOEWE cluster in CSC in Frankfurt, and on the HazelHen cluster at the HLRS in Stuttgart. This research was enabled in part by support provided by Compute Ontario (http://computeontario. r ca), Calcul Quebec (http://www.calculquebec.ca) and Compute Canada (http://www.computecanada.ca).We thank the sta ff at the participating observatories, correlation centers, and institutions for their enthusiastic support. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2016.1.01154.V, ADS/JAO.ALMA#2016.1.01176.V. ALMA is a partnership of the European Southern Observatory (ESO; Europe, representing its member states), NSF, and National Institutes of Natural Sciences of Japan, together with National Research Council (Canada), Ministry of Science and Technology (MOST; Taiwan), Academia Sinica Institute of Astronomy and Astrophysics (ASIAA; Taiwan), and Korea Astronomy and Space Science Institute (KASI; Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, Associated Universities, Inc. (AUI)/NRAO, and the National Astronomical Observatory of Japan (NAOJ). The NRAO is a facility of the NSF operated under cooperative agreement by AUI. APEX is a collaboration between the Max-Planck-Institut fur Radioastronomie (Germany), ESO, and the Onsala Space Observatory (Sweden). The SMA is a joint project between the SAO and ASIAA and is funded by the Smithsonian Institution and the Academia Sinica. The JCMT is operated by the East Asian Observatory on behalf of the NAOJ, ASIAA, and KASI, as well as the Ministry of Finance of China, Chinese Academy of Sciences, and the National Key R&D Program (No. 2017YFA0402700) of China. Additional funding support for the JCMT is provided by the Science and Technologies Facility Council (UK) and participating universities in the UK and Canada. The LMT is a project operated by the Instituto Nacional de Astrofisica, Optica, y Electronica (Mexico) and the University of Massachusetts at Amherst (USA). The IRAM 30-m telescope on Pico Veleta, Spain is operated by IRAM and supported by CNRS (Centre National de la Recherche Scientifique, France), MPG (Max-Planck-Gesellschaft, Germany) and IGN (Instituto Geografico Nacional, Spain). The SMT is operated by the Arizona Radio Observatory, a part of the Steward Observatory of the University of Arizona, with financial support of operations from the State of Arizona and financial support for instrumentation development from the NSF. The SPT is supported by the National Science Foundation through grant PLR1248097. Partial support is also provided by the NSF Physics Frontier Center grant PHY-1125897 to the Kavli Institute of Cosmological Physics at the University of Chicago, the Kavli Foundation and the Gordon and Betty Moore Foundation grant GBMF 947. The SPT hydrogen maser was provided on loan from the GLT, courtesy of ASIAA. The EHTC has received generous donations of FPGA chips from Xilinx Inc., under the Xilinx University Program. The EHTC has benefited from technology shared under open-source license by the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER). The EHT project is grateful to T4Science and Microsemi for their assistance with Hydrogen Masers. This research has made use of NASA's Astrophysics Data System. We gratefully acknowledge the support provided by the extended sta ff of the ALMA, both from the inception of the ALMA Phasing Project through the observational campaigns of 2017 and 2018. We would like to thank A. Deller and W. Brisken for EHT-specific support with the use of DiFX. We acknowledge the significance that Maunakea, where the SMA and JCMT EHT stations are located, has for the indigenous Hawaiian people. r This research has made use of data obtained with the Global Millimeter VLBI Array (GMVA), which consists of telescopes operated by the MPIfR, IRAM, Onsala, Metsahovi, Yebes, the Korean VLBI Network, the Green Bank Observatory and the Very Long Baseline Array (VLBA). The VLBA is an instrument of the National Radio Astronomy Observatory. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated by Associated Universities, Inc. The data were correlated at the correlator of the MPIfR in Bonn, Germany. This study makes use of 43 GHz VLBA data from the VLBA-BU Blazar Monitoring Program (VLBABU-BLAZAR; http://www.bu.edu/blazars/VLBAproject.html), funded by NASA through the Fermi Guest Investigator Program.