# First Very Long Baseline Interferometry Detections at 870µm

Alexander W Raymond, Sheperd S Doeleman, Keiichi Asada, Lindy

Blackburn, Geoffrey C Bower, Michael Bremer, Dominique Broguiere,
Ming-Tang Chen, Geoffrey B Crew, Sven Dornbusch, et al.

► To cite this version:

Alexander W Raymond, Sheperd S Doeleman, Keiichi Asada, Lindy Blackburn, Geoffrey C Bower, et al.. First Very Long Baseline Interferometry Detections at 870µm. Astron.J., 2024, 168 (3), pp.130. 10.3847/1538-3881/ad5bdb. hal-04736801

HAL Id: hal-04736801

https://hal.science/hal-04736801v1

Submitted on 17 Oct 2024

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Distributed under a Creative Commons Attribution 4.0 International License
First Very Long Baseline Interferometry Detections at 870 µm

Alexander W. Raymond¹,²,³, Sheperd S. Doeleman¹,²,¹⁵¹, Keiichi Asada⁴, Lindy Blackburn¹,², Geoffrey C. Bower⁵,⁶, Michael Bremer⁷, Dominique Broguiere⁷, Ming-Tang Chen⁴, Geoffrey B. Crew⁸, Sven Dornbusch⁹, Vincent L. Fish⁸, Roberto García⁷, Olivier Gentaz⁷, Ciriaco Goddi¹⁰,¹¹, Chih-Chiang Han⁴, Michael H. Hecht⁸, Yau-De Huang⁴, Michael Janssen¹⁸,⁹, Garrett K. Keating², Jun Yi Koay⁴, Thomas P. Krichbaum⁹, Wen-Ping Lo¹,⁴,¹², Satoki Matsushita⁴, Lynn D. Matthews⁸, James M. Moran¹,², Timothy J. Norton², Nimesh Patel², Dominic W. Pesce¹,², Venkatessh Ramakrishnan¹³,¹⁴,¹⁵, Helge Rottmann⁹, Alan L. Roy⁹, Salvador Sánchez¹⁶, Remo P. J. Tilanus¹⁷, Michael Titus⁸, Pablo Torne⁹,¹⁶, Jan Wagner⁹, Jonathan Weintroub¹,², Maciek Wielgus⁹, André Young¹⁸, Kazunori Akiyama¹,⁸,¹⁹, Ezequiel Albentosa-Ruiz²⁰, Antxon Alberdi²¹, Walter Alef⁹, Juan Carlos Algaba²², Richard Anantua¹,²,²³, Rebecca Azulay⁹,²⁰,²⁴, Uwe Bach⁹, Anne-Kathrin Baczko⁹,²⁵, David Ball¹⁷, Mislav Balokovic²⁶, Bidisha Bandyopadhyay¹³, John Barrett⁸, Michi Bauböck²⁷, Bradford A. Benson²⁸,²⁹, Dan Bintley³⁰,³¹, Raymond Blundell², Katherine L. Bouman³², Hope Boyce³³,³⁴, Roger Brissenden¹,², Silke Britzen⁹, Avery E. Broderick³⁵,³⁶,³⁷, Thomas Bronzwaer¹⁸, Sandra Bustamante³⁸, John E. Carlstrom²⁹,³⁹,⁴⁰,⁴¹, Andrew Chael⁴², Chi-kwan Chan¹⁷,⁴³,⁴⁴, Dominic O. Chang¹,², Koushik Chatterjee¹,², Shami Chatterjee⁴⁵, Yongjun Chen (陈永军)⁴⁶,⁴⁷, Xiaopeng Cheng⁴⁸, Ilje Cho²¹,⁴⁸,⁴⁹, Pierre Christian⁵⁰, Nicholas S. Conroy²,⁵¹, John E. Conway²⁵, Thomas M. Crawford²⁹,³⁹, Alejandro Cruz-Osorio⁵²,⁵³, Yuzhu Cui (崔玉竹)⁵⁴,⁵⁵, Rohan Dahale²¹, Jordy Davelaar¹⁸,⁵⁶,⁵⁷, Mariafelicia De Laurentis⁵⁸,⁵⁹, Roger Deane⁶⁰,⁶¹,⁶², Jessica Dempsey³⁰,³¹,⁶³, Gregory Desvignes⁹,⁶⁴, Jason Dexter⁶⁵, Vedant Dhruv²⁷, Indu K. Dihingia⁵⁵, Sergio A. Dzib⁹,⁷, Ralph P. Eatough⁹,⁶⁶, Razieh Emami², Heino Falcke¹⁸, Joseph Farah⁶⁷,⁶⁸, Edward Fomalont⁶⁹, Anne-Laure Fontana⁷, H. Alyson Ford¹⁷, Marianna Foschi²¹, Raquel Fraga-Encinas¹⁸, William T. Freeman⁷⁰,⁷¹, Per Friberg³⁰,³¹, Christian M. Fromm⁹,⁵³,⁷², Antonio Fuentes²¹, Peter Galison¹,⁷³,⁷⁴, Charles F. Gammie²⁷,⁵¹,⁷⁵, Boris Georgiev¹⁷, Roman Gold⁷⁶, Arturo I. Gómez-Ruiz⁷⁷,⁷⁸, José L. Gómez²¹, Minfeng Gu (顾敏峰)⁴⁶,⁷⁹, Mark Gurwell², Kazuhiro Hada⁸⁰,⁸¹, Daryl Haggard³³,³⁴, Ronald Hesper⁸², Dirk Heumann¹⁷, Luis C. Ho (何子山)⁸³,⁸⁴, Paul Ho⁴,³⁰,³¹, Mareki Honma⁸⁰,⁸¹,⁸⁵, Chih-Wei L. Huang⁴, Lei Huang (黄磊)⁴⁶,⁷⁹, David H. Hughes⁷⁷, Shiro Ikeda¹⁹,⁸⁶,⁸⁷,⁸⁸, C. M. Violette Impellizzeri⁶⁹,⁸⁹, Makoto Inoue⁴, Sara Issaoun²,¹⁵⁰, David J. James⁹⁰,⁹¹, Buell T. Jannuzi¹⁷, Britton Jeter⁴, Wu Jiang (江悟)⁴⁶, Alejandra Jiménez-Rosales¹⁸, Michael D. Johnson¹,², Svetlana Jorstad⁹², Adam C. Jones²⁹, Abhishek V. Joshi²⁷, Taehyun Jung⁴⁸,⁹³, Ramesh Karuppusamy⁹, Tomohisa Kawashima⁹⁴, Mark Kettenis⁹⁵, Dong-Jin Kim⁸, Jae-Young Kim⁹,⁹⁶, Jongsoo Kim⁴⁸, Junhan Kim⁹⁷, Motoki Kino¹⁹,⁹⁸, Prashant Kocherlakota⁵³, Yutaro Kofuji⁸⁰,⁸⁵, Patrick M. Koch⁴, Shoko Koyama⁴,⁹⁹, Carsten Kramer⁷, Joana A. Kramer⁹, Michael Kramer⁹, Derek Kubo⁵, Cheng-Yu Kuo⁴,¹⁰⁰, Noemi La Bella¹⁸, Sang-Sung Lee⁴⁸, Aviad Levis³², Zhiyuan Li (李志远)¹⁰¹,¹⁰², Rocco Lico²¹,¹⁰³, Greg Lindahl², Michael Lindqvist²⁵, Mikhail Lisakov⁹, Jun Liu (刘俊)⁹, Kuo Liu⁹, Elisabetta Liuzzo¹⁰⁴, Andrei P. Lobanov⁹, Laurent Loinard¹⁰⁵, Colin J. Lonsdale⁸, Amy E. Lowitz¹⁷, Ru-Sen Lu (路如森)⁹,⁴⁶,⁴⁷, Nicholas R. MacDonald⁹, Sylvain Mahieu⁷, Doris Maier⁷, Jirong Mao (毛基荣)¹⁰⁶,¹⁰⁷,¹⁰⁸, Nicola Marchili⁹,¹⁰⁴, Sera Markoff¹⁰⁹,¹¹⁰, Daniel P. Marrone¹⁷, Alan P. Marscher⁹², Iván Martí-Vidal²⁰,²⁴, Lia Medeiros¹¹¹,¹⁵⁰, Karl M. Menten⁹, Izumi Mizuno³⁰,³¹, Yosuke Mizuno⁵³,⁵⁵,¹¹², Joshua Montgomery²⁹,³⁴, Kotaro Moriyama⁵³,⁸⁰, Monika Moscibrodzka¹⁸, Wanga Mulaudzi¹⁰⁹, Cornelia Müller⁹,¹⁸, Hendrik Müller⁹, Alejandro Mus²⁰,²⁴, Gibwa Musoke¹⁸,¹⁰⁹, Ioannis Myserlis¹⁶, Hiroshi Nagai¹⁹,⁸¹, Neil M. Nagar¹³, Masanori Nakamura⁴,¹¹³, Gopal Narayanan³⁸, Iniyan Natarajan¹,², Antonios Nathanail⁵³,¹¹⁴, Santiago Navarro Fuentes¹⁶, Joey Neilsen¹¹⁵, Chunchong Ni³⁵,³⁶,³⁷, Michael A. Nowak¹¹⁶, Junghwan Oh⁹⁵, Hiroki Okino⁸⁰,⁸⁵, Héctor Raúl Olivares Sánchez¹¹⁷, Tomoaki Oyama⁸⁰, Feryal Özel¹¹⁸, Daniel C. M. Palumbo¹,², Georgios Filippos Paraschos⁹, Jongho Park¹¹⁹, Harriet Parsons³⁰,³¹, Ue-Li Pen⁴,³⁵,¹²⁰,¹²¹,¹²², Vincent Piétu⁷, Aleksandar PopStefanija³⁸, Oliver Porth⁵³,¹⁰⁹, Ben Prather²⁷, Giacomo Principe¹⁰³,¹²³,¹²⁴, Dimitrios Psaltis¹¹⁸, Hung-Yi Pu⁴,¹²⁵,¹²⁶, Philippe A. Raffin⁴, Ramprasad Rao², Mark G. Rawlings³⁰,³¹,¹²⁷, Angelo Ricarte¹,², Bart Ripperda³⁵,¹²⁰,¹²¹,¹²⁸, Freek Roelofs¹,²,¹⁸, Cristina Romero-Canizales⁴, Eduardo Ros⁹, Arash Roshanineshat¹⁷, Ignacio Ruiz¹⁶, Chet Ruszczyk⁸, Kazi L. J. Rygl¹⁰⁴, David Sánchez-Argüelles⁷⁷,⁷⁸, Miguel Sánchez-Portal¹⁶, Mahito Sasada⁸⁰,¹²⁹,¹³⁰, Kaushik Satapathy¹⁷, Tuomas Savolainen⁹,¹⁵,¹³¹, F. Peter Schloerb³⁸, Jonathan Schonfeld², Karl-Friedrich Schuster⁷, Lijing Shao⁹,⁸⁴, Zhiqiang Shen (沈志强)⁴⁶,⁴⁷, Des Small⁹⁵, Bong Won Sohn⁴⁸,⁴⁹,⁹³, Jason SooHoo⁸, León David Sosapanta Salas¹⁰⁹, Kamal Souccar³⁸, Ranjani Srinivasan², Joshua S. Stanway¹³², He Sun (孙赫)¹³³,¹³⁴, Fumie Tazaki¹³⁵, Alexandra J. Tetarenko¹³⁶, Paul Tiede¹,², Kenji Toma¹³⁷,¹³⁸, Teresa Toscano²¹, Efthalia Traianou⁹,²¹, Tyler Trent¹⁷, Sascha Trippe¹³⁹, Matthew Turk⁵¹, Ilse van Bemmel⁹⁵, Huib Jan van Langevelde⁸⁹,⁹⁵,¹⁴⁰, Daniel R. van Rossum¹⁸, Jesse Vos¹⁸, Derek Ward-Thompson¹³², John Wardle¹⁴¹, Jasmin E. Washington¹⁷, Robert Wharton⁹, Kaj Wiik¹⁴², Gunther Witzel⁹, Michael F. Wondrak¹⁸,¹⁴³,
George N. Wong<sup>42,144</sup>①, Qingwen Wu (吴庆文)<sup>145</sup>①, Nitika Yadlapalli<sup>32</sup>①, Paul Yamaguchi<sup>2</sup>①, Aristomenis Yfantis<sup>18</sup>①,
Doosoo Yoon<sup>109</sup>①, Ziri Younsi<sup>53,146</sup>①, Wei Yu (于威)<sup>2</sup>①, Feng Yuan (袁峰)<sup>147</sup>①, Ye-Fei Yuan (袁业飞)<sup>148</sup>①,
J. Anton Zensus<sup>9</sup>①, Shuo Zhang<sup>149</sup>①, Guang-Yao Zhao<sup>5,21</sup>①, and Shan-Shan Zhao (赵杉杉)<sup>46</sup>①

¹ Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA; sdoeleman@cfa.harvard.edu

²Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

³Current address is Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

⁴Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Sec. 4, Roosevelt Road, Taipei 106216, Taiwan, R.O.C.

⁵Institute of Astronomy and Astrophysics, Academia Sinica, 645 North A'ohoku Place, Hilo, HI 96720, USA

⁶Department of Physics and Astronomy, University of Hawaii at Manoa, 2505 Correa Road, Honolulu, HI 96822, USA

⁷Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, F-38406 Saint Martin d'Hères, France

⁸Massachusetts Institute of Technology Haystack Observatory, 99 Millstone Road, Westford, MA 01886, USA

⁹Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

¹⁰Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu km 0.7, I-09042 Monserrato, Italy

¹¹INAF-Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047, Selargius, CA, Italy

¹²Department of Physics, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 106216, Taiwan, R.O.C.

¹³Astronomy Department, Universidad de Concepción, Casilla 160-C, Concepción, Chile

¹⁴Finnish Centre for Astronomy with ESO, FI-20014 University of Turku, Finland

¹⁵Aalto University Metsähovi Radio Observatory, Metsähovintie 114, FI-02540 Kylmälä, Finland

¹⁶Institut de Radioastronomie Millimétrique (IRAM), Avenida Divina Pastora 7, Local 20, E-18012, Granada, Spain

¹⁷Steward Observatory and Department of Astronomy, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA

¹⁸Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands

¹⁹National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

²⁰Department d'Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, E-46100 Burjassot, València, Spain

²¹Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain

²²Department of Physics, Faculty of Science, Universiti Malaya, 50603 Kuala Lumpur, Malaysia

²³Department of Physics & Astronomy, The University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA

²⁴Observatori Astronòmic, Universitat de València, C. Catedrático José Beltrán 2, E-46980 Paterna, València, Spain

²⁵Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, SE-43992 Onsala, Sweden

²⁶Yale Center for Astronomy & Astrophysics, Yale University, 52 Hillhouse Avenue, New Haven, CT 06511, USA

²⁷Department of Physics, University of Illinois, 1110 West Green Street, Urbana, IL 61801, USA

²⁸Fermi National Accelerator Laboratory, MS209, P.O. Box 500, Batavia, IL 60510, USA

²⁹Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA

³⁰East Asian Observatory, 660 North A'ohoku Place, Hilo, HI 96720, USA

³¹James Clerk Maxwell Telescope (JCMT), 660 North A'ohoku Place, Hilo, HI 96720, USA

³²California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA

³³Department of Physics, McGill University, 3600 rue University, Montréal, QC H3A 2T8, Canada

³⁴Trottier Space Institute at McGill, 3550 rue University, Montréal, QC H3A 2A7, Canada

³⁵Perimeter Institute for Theoretical Physics, 31 Caroline Street North, Waterloo, ON N2L 2Y5, Canada

³⁶Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada

³⁷Waterloo Centre for Astrophysics, University of Waterloo, Waterloo, ON N2L 3G1, Canada

³⁸Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA

³⁹Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA

⁴⁰Department of Physics, University of Chicago, 5720 South Ellis Avenue, Chicago, IL 60637, USA

⁴¹Enrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA

⁴²Princeton Gravity Initiative, Jadwin Hall, Princeton University, Princeton, NJ 08544, USA

⁴³Data Science Institute, University of Arizona, 1230 North Cherry Avenue, Tucson, AZ 85721, USA

⁴⁴Program in Applied Mathematics, University of Arizona, 617 North Santa Rita, Tucson, AZ 85721, USA

⁴⁵Cornell Center for Astrophysics and Planetary Science, Cornell University, Ithaca, NY 14853, USA

⁴⁶Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, People's Republic of China

⁴⁷Key Laboratory of Radio Astronomy and Technology, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing, 100101, People's Republic of China

⁴⁸Korea Astronomy and Space Science Institute, Daedeok-daero 776, Yuseong-gu, Daejeon 34055, Republic of Korea

⁴⁹Department of Astronomy, Yonsei University, Yonse-ro 50, Seodaemun-gu, 03722 Seoul, Republic of Korea

⁵⁰Physics Department, Fairfield University, 1073 North Benson Road, Fairfield, CT 06824, USA

⁵¹Department of Astronomy, University of Illinois at Urbana-Champaign, 1002 West Green Street, Urbana, IL 61801, USA

⁵²Instituto de Astronomia, Universidad Nacional Autónoma de México (UNAM), Apdo Postal 70-264, Ciudad de México, Mexico

⁵³Institut für Theoretische Physik, Goethe-Universität Frankfurt, Max-von-Laue-Straße 1, D-60438 Frankfurt am Main, Germany

⁵⁴Research Center for Astronomical Computing, Zhejiang Laboratory, Hangzhou 311100, People's Republic of China

⁵⁵Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shengrong Road 520, Shanghai, 201210, People's Republic of China

⁵⁶Department of Astronomy and Columbia Astrophysics Laboratory, Columbia University, 500 West 120th Street, New York, NY 10027, USA

⁵⁷Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA

⁵⁸Dipartimento di Fisica “E. Pancini”, Università di Napoli “Federico II”, Compl. Univ. di Monte S. Angelo, Edificio G, Via Cinthia, I-80126, Napoli, Italy

⁵⁹INFN Sez. di Napoli, Compl. Univ. di Monte S. Angelo, Edificio G, Via Cinthia, I-80126, Napoli, Italy

⁶⁰Wits Centre for Astrophysics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg 2050, South Africa

⁶¹Department of Physics, University of Pretoria, Hatfield, Pretoria 0028, South Africa

⁶²Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, Makhanda 6140, South Africa

⁶³ASTRON, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands

⁶⁴LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, F-92195 Meudon, France

⁶⁵JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309, USA

⁶⁶National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100101, People's Republic of China

⁶⁷Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117-5575, USA

⁶⁸Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA

⁶⁹National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁷⁰ Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 32-D476, 77 Massachusetts Avenue, Cambridge, MA 02142, USA

⁷¹ Google Research, 355 Main Street, Cambridge, MA 02142, USA

⁷² Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Emil-Fischer-Str. 31, D-97074 Würzburg, Germany

⁷³ Department of History of Science, Harvard University, Cambridge, MA 02138, USA

⁷⁴ Department of Physics, Harvard University, Cambridge, MA 02138, USA

⁷⁵ NCSA, University of Illinois, 1205 West Clark Street, Urbana, IL 61801, USA

⁷⁶ CP3-Origins, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark

⁷⁷ Instituto Nacional de Astrofísica, Óptica y Electrónica. Apartado Postal 51 y 216, 72000. Puebla Pue., Mexico

⁷⁸ Consejo Nacional de Humanidades, Ciencia y Tecnología, Av. Insurgentes Sur 1582, 03940, Ciudad de México, Mexico

⁷⁹ Key Laboratory for Research in Galaxies and Cosmology, Chinese Academy of Sciences, Shanghai 200030, People's Republic of China

⁸⁰ Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Mizusawa, Oshu, Iwate 023-0861, Japan

⁸¹ Department of Astronomical Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

⁸² NOVA Sub-mm Instrumentation Group, Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands

⁸³ Department of Astronomy, School of Physics, Peking University, Beijing 100871, People's Republic of China

⁸⁴ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People's Republic of China

⁸⁵ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

⁸⁶ The Institute of Statistical Mathematics, 10-3 Midori-cho, Tachikawa, Tokyo, 190-8562, Japan

⁸⁷ Department of Statistical Science, The Graduate University for Advanced Studies (SOKENDAI), 10-3 Midori-cho, Tachikawa, Tokyo 190-8562, Japan

⁸⁸ Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, 277-8583, Japan

⁸⁹ Leiden Observatory, Leiden University, Postbus 2300, 9513 RA Leiden, The Netherlands

⁹⁰ ASTRAVEO LLC, PO Box 1668, Gloucester, MA 01931, USA

⁹¹ Applied Materials Inc., 35 Dory Road, Gloucester, MA 01930, USA

⁹² Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA

⁹³ University of Science and Technology, Gajeong-ro 217, Yuseong-gu, Daejeon 34113, Republic of Korea

⁹⁴ Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8582, Japan

⁹⁵ Joint Institute for VLBI ERIC (JIVE), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands

⁹⁶ Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu 702-701, Republic of Korea

⁹⁷ Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

⁹⁸ Kogakuin University of Technology & Engineering, Academic Support Center, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan

⁹⁹ Graduate School of Science and Technology, Niigata University, 8050 Ikarashi 2-no-cho, Nishi-ku, Niigata 950-2181, Japan

¹⁰⁰ Physics Department, National Sun Yat-Sen University, No. 70, Lien-Hai Road, Kaosiung City 80424, Taiwan, R.O.C.

¹⁰¹ School of Astronomy and Space Science, Nanjing University, Nanjing 210023, People's Republic of China

¹⁰² Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Nanjing 210023, People's Republic of China

¹⁰³ INAF-Istituto di Radioastronomia, Via P. Gobetti 101, I-40129 Bologna, Italy

¹⁰⁴ INAF-Istituto di Radioastronomia & Italian ALMA Regional Centre, Via P. Gobetti 101, I-40129 Bologna, Italy

¹⁰⁵ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Morelia 58089, Mexico

¹⁰⁶ Yunnan Observatories, Chinese Academy of Sciences, 650011 Kunming, Yunnan Province, People's Republic of China

¹⁰⁷ Center for Astronomical Mega-Science, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing, 100012, People's Republic of China

¹⁰⁸ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, 650011 Kunming, People's Republic of China

¹⁰⁹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands

¹¹⁰ Gravitation and Astroparticle Physics Amsterdam (GRAPPA) Institute, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

¹¹¹ Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, NJ 08544, USA

¹¹² School of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, People's Republic of China

¹¹³ National Institute of Technology, Hachinohe College, 16-1 Uwanotai, Tamonoki, Hachinohe City, Aomori 039-1192, Japan

¹¹⁴ Research Center for Astronomy, Academy of Athens, Soranou Efessiou 4, 115 27 Athens, Greece

¹¹⁵ Department of Physics, Villanova University, 800 Lancaster Avenue, Villanova, PA 19085, USA

¹¹⁶ Physics Department, Washington University, CB 1105, St. Louis, MO 63130, USA

¹¹⁷ Departamento de Matemática da Universidade de Aveiro and Centre for Research and Development in Mathematics and Applications (CIDMA), Campus de Santiago, 3810-193 Aveiro, Portugal

¹¹⁸ School of Physics, Georgia Institute of Technology, 837 State Street NW, Atlanta, GA 30332, USA

¹¹⁹ School of Space Research, Kyung Hee University, 1732, Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104, Republic of Korea

¹²⁰ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada

¹²¹ Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada

¹²² Canadian Institute for Advanced Research, 180 Dundas Street West, Toronto, ON M5G 1Z8, Canada

¹²³ Dipartimento di Fisica, Università di Trieste, I-34127 Trieste, Italy

¹²⁴ INFN Sez. di Trieste, I-34127 Trieste, Italy

¹²⁵ Department of Physics, National Taiwan Normal University, No. 88, Sec. 4, Tingzhou Road, Taipei 116, Taiwan, R.O.C.

¹²⁶ Center of Astronomy and Gravitation, National Taiwan Normal University, No. 88, Sec. 4, Tingzhou Road, Taipei 116, Taiwan, R.O.C.

¹²⁷ Gemini Observatory/NSF's NOIRLab, 670 North A'ohoku Place, Hilo, HI 96720, USA

¹²⁸ Department of Physics, University of Toronto, 60 St. George Street, Toronto, ON M5S 1A7, Canada

¹²⁹ Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan

¹³⁰ Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan

¹³¹ Aalto University Department of Electronics and Nanoengineering, PL 15500, FI-00076 Aalto, Finland

¹³² Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK

¹³³ National Biomedical Imaging Center, Peking University, Beijing 100871, People's Republic of China

¹³⁴ College of Future Technology, Peking University, Beijing 100871, People's Republic of China

¹³⁵ Tokyo Electron Technology Solutions Limited, 52 Matsunagane, Iwayado, Esashi, Oshu, Iwate 023-1101, Japan

¹³⁶ Department of Physics and Astronomy, University of Lethbridge, Lethbridge, AB T1K 3M4, Canada

¹³⁷ Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980-8578, Japan

¹³⁸ Astronomical Institute, Tohoku University, Sendai 980-8578, Japan

¹³⁹ Department of Physics and Astronomy, Seoul National University, Gwanak-gu, Seoul 08826, Republic of Korea

¹⁴⁰ University of New Mexico, Department of Physics and Astronomy, Albuquerque, NM 87131, USA

¹⁴¹ Physics Department, Brandeis University, 415 South Street, Waltham, MA 02453, USA

¹⁴² Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Finland

¹⁴³ Radboud Excellence Fellow of Radboud University, Nijmegen, The Netherlands
Received 2024 April 30; revised 2024 June 23; accepted 2024 June 24; published 2024 August 27

Abstract

The first very long baseline interferometry (VLBI) detections at 870 µm wavelength (345 GHz frequency) are reported, achieving the highest diffraction-limited angular resolution yet obtained from the surface of the Earth and the highest-frequency example of the VLBI technique to date. These include strong detections for multiple sources observed on intercontinental baselines between telescopes in Chile, Hawaii, and Spain, obtained during observations in 2018 October. The longest-baseline detections approach 11 Gλ, corresponding to an angular resolution, or fringe spacing, of 19 µas. The Allan deviation of the visibility phase at 870 µm is comparable to that at 1.3 mm on the relevant integration timescales between 2 and 100 s. The detections confirm that the sensitivity and signal chain stability of stations in the Event Horizon Telescope (EHT) array are suitable for VLBI observations at 870 µm. Operation at this short wavelength, combined with anticipated enhancements of the EHT, will lead to a unique high angular resolution instrument for black hole studies, capable of resolving the event horizons of supermassive black holes in both space and time.

*Unified Astronomy Thesaurus concepts: Very long baseline interferometry (1769); Radio interferometry (1346); Black holes (162); Supermassive black holes (1663); High angular resolution (2167); Astronomical techniques (1684); Event horizons (479)*

1. Introduction

The technique of very long baseline interferometry (VLBI) involves a network of independently clocked telescopes separated by large distances, which simultaneously observe a common astronomical source (Thompson et al. 2017). The angular resolution, or fringe spacing, in a VLBI observation scales inversely with both the distance between stations (i.e., the length of the baseline) and the observing frequency. The present article reports the first fringe detections made at 870 µm wavelength (345 GHz nominal frequency), which constitutes the shortest-wavelength VLBI observation to date. The experiment we describe was intended as a first technical demonstration of the 870 µm VLBI capability using facilities that are part of the Event Horizon Telescope (EHT) array. Figure 1 shows the stations that participated in the fringe test along with the usual metric used to characterize millimeter-wavelength observing conditions: the 225 GHz zenith opacity (Thompson et al. 2017).

The VLBI observing wavelength has decreased over time. The first 3 mm VLBI detections (at 86 GHz) were obtained through observations performed in 1981 (Readhead et al. 1983), the first 3 mm intercontinental detections (100 GHz) were obtained through observations performed in 1988 (Baath et al. 1991, 1992), and the first successful 1.3 mm (230 GHz) VLBI was carried out in 1989 (Padin et al. 1990). The especially long time since the last significant decrease in VLBI wavelength reflects the challenges of carrying out such observations, which are detailed below. Even so, there have been several milestones of note since the early 1990s on the path toward developing short-wavelength VLBI as an

important technique for astrophysics. Increased sensitivity through the use of larger telescopes and advanced receivers led to 1.4 mm (215 GHz) detections on a ~1100 km baseline of multiple active galactic nuclei (AGN) and Sagittarius A* (Sgr A*), the Galactic center supermassive black hole (Greve et al. 1995; Krichbaum et al. 1997, 1998). A return to the longer-wavelength 2 mm spectral windows (147 GHz and 129 GHz) allowed extension of millimeter-wavelength VLBI to intercontinental baselines (Doeleman et al. 2002; Greve et al. 2002; Krichbaum et al. 2002). Building on this work, Doeleman et al. (2008, 2012) used purpose-built wideband digital VLBI systems on 1.3 mm transoceanic baselines to report the discovery of event-horizon-scale structures in Sgr A* and the much more massive black hole, M87*. The EHT collaboration has now imaged both of these sources with a global 1.3 mm VLBI array (Event Horizon Telescope Collaboration et al. 2019a, 2022a, 2024).

The EHT is the highest-resolution ground-based VLBI instrument to date (Event Horizon Telescope Collaboration et al. 2019b). The EHT fringe spacing is approximately 25 µas at 1.3 mm wavelength. The finite diameter of the Earth limits ground-based 1.3 mm fringe spacing to 21 µas, corresponding to a 9.8 Gλ baseline. In practice, modern imaging methods, such as regularized maximum likelihood, achieve a slightly higher angular resolution that exceeds the diffraction limit (Event Horizon Telescope Collaboration et al. 2019c).

For future campaigns, the EHT has developed the capability to observe at 870 µm, and enhancing the ability to observe at this wavelength through new stations and wider bandwidth is an important aspect of long-term enhancements envisaged by the next-generation EHT (ngEHT) project (Doeleman et al. 2019, 2023; Raymond et al. 2021). For a given set of station locations, observing at 870 µm improves angular resolution by approximately 50% compared to observing at 1.3 mm, which will provide a sharper view of the black hole shadow and environment; the 870 µm fringe spacing limit set by the diameter of the Earth is approximately 14 µas, corresponding to a 14.7 Gλ baseline. Observations at 870 µm are also important

<figure><img src="image_2.png" /></figure>

Original content from this work may be used under the terms of the [Creative Commons Attribution 4.0 licence](https://creativecommons.org/licenses/by/4.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="144">¹⁴⁴ School of Natural Sciences, Institute for Advanced Study, 1 Einstein Drive, Princeton, NJ 08540, USA</footnote>

<footnote n="145">¹⁴⁵ School of Physics, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, People's Republic of China</footnote>

<footnote n="146">¹⁴⁶ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK</footnote>

<footnote n="147">¹⁴⁷ Center for Astronomy and Astrophysics and Department of Physics, Fudan University, Shanghai 200438, People's Republic of China</footnote>

<footnote n="148">¹⁴⁸ Astronomy Department, University of Science and Technology of China, Hefei 230026, People's Republic of China</footnote>

<footnote n="149">¹⁴⁹ Department of Physics and Astronomy, Michigan State University, 567 Wilson Road, East Lansing, MI 48824, USA</footnote>

<footnote n="150">¹⁵⁰ NASA Hubble Fellowship Program, Einstein Fellow.</footnote>

<footnote n="151">¹⁵¹ Corresponding author.</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 1.** (Top) Stations in the 870 µm fringe test. (Bottom) Zenith opacity at 225 GHz, which is the standard frequency used for monitoring millimeter-wave conditions. The observing window on each day is indicated by the green shading. Conditions at ALMA were very good during both days ($\tau_{225} \approx 0.05$). The black lines indicate the opacity at each site calculated using inputs from MERRA-2 reanalysis during the observing windows, which we use to estimate 870 µm (345 GHz) opacity. Opacities for APEX and NOEMA have been estimated by converting precipitable water vapor column amounts.</figcaption></figure>

for polarimetric measurements. Faraday rotation, which scrambles the imaged electric field vector position angle pattern, diminishes with the square of the frequency. Therefore, 870 µm observations may help distinguish Faraday rotation from the intrinsic field pattern set by the horizon-scale magnetic field and plasma properties (Event Horizon Telescope Collaboration et al. 2021; Wielgus et al. 2024). For Sgr A*, the angular size of the black hole shadow is larger than that of M87* (Event Horizon Telescope Collaboration et al. 2022a), but scattering in the ionized interstellar medium affects the image angular resolution (see, e.g., Johnson et al. 2018). At 1.3 mm, the scatter broadening is comparable to the current EHT resolution, but it decreases approximately as the observing wavelength squared. Thus, at 870 µm, scattering

effects would be significantly diminished and would not limit the resolution of a VLBI array for studies of Sgr A*. In particular, extension of the EHT to 870 µm wavelengths can target photon ring substructure in Sgr A*, aiming to detect the orbit of light that makes a full “u-turn” around the black hole (Johnson et al. 2020; Palumbo et al. 2023). For these reasons, 870 µm VLBI opens important new directions for advanced horizon-resolved studies of the two primary EHT sources. At the same time, higher-frequency VLBI brings more sources into range for horizon-resolved black hole studies (Pesce et al. 2021; Lo et al. 2023; Ramakrishnan et al. 2023), and the increased resolution at 870 µm benefits nonhorizon VLBI studies of AGN jets (e.g., Kim et al. 2020; Janssen et al. 2021; Issaoun et al. 2022; Jorstad et al. 2023; Paraschos et al. 2024). Additionally, due to reduced opacity, shorter wavelengths probe more compact regions of jetted AGN sources (an example being the core-shift effect; Lobanov 1998; Hada et al. 2011). Hence, 870 µm VLBI has the potential to image the jet launching region closer to the central black hole, enabling investigations of the physics behind jet formation, collimation, and acceleration. In particular, the poorly understood limb brightening in transversely resolved inner jets (e.g., Janssen et al. 2021) can be studied in much greater detail.

Extension of observing to 870 µm similarly enhances the capability of the EHT to capture dynamics near the event horizon. In the case of Sgr A*, the dynamical timescale is ~200 s (10GM/c³). Simultaneous 1.3 mm and 870 µm observing can sample sufficient Fourier spatial frequencies within this integration time to allow snapshot imaging using the technique of multifrequency synthesis (MFS; Chael et al. 2023). Combining such snapshots will enable recovery of accretion and jet launching kinematics. For M87*, the dynamical timescale is ~3 days, and data obtained in both 1.3 mm and 870 µm on sequential days can be combined to form high-fidelity MFS images for time-lapse movie reconstruction of the event horizon environment. Realizing the full scientific potential of 870 µm VLBI (Johnson et al. 2023) will require the planned ngEHT upgrade (Doeleman et al. 2023).

While there are clearly many motivating reasons for 870 µm VLBI observing, a number of factors make the measurements difficult in this short-wavelength regime. The atmosphere is more opaque at 870 µm than at 1.3 mm (see, for example, Liebe 1985; Matsushita et al. 1999, 2016, 2022), which means that sources are more attenuated and noise levels due to atmospheric emission are elevated. Overall, the effective system temperatures of coherent radio receivers are intrinsically greater at 870 µm than at 1.3 mm.<fnref n="152" /> The aperture efficiency of the collecting optics tends to diminish at high frequency, and the source flux density tends to decrease. In addition, coherence losses due to the VLBI frequency standards used at each site increase with observing frequency (Doeleman et al. 2011). The EHT array, conceived as a common, international effort of independent observatories working in the short-millimeter range, has directly addressed these challenges and provides key enabling infrastructure for extension of VLBI to higher frequencies (Event Horizon Telescope Collaboration et al. 2019b).

The telescopes comprising the EHT array are precision structures sited at high-altitude, low-opacity locations (see, e.g., Levy et al. 1996; Mangum et al. 2006; Greve & Bremer 2010; Chen et al. 2023 and references therein on the design and

<footnote n="152">¹⁵² See, for example, Janssen et al. (2019) or the ALMA Cycle 8 2021 Technical Handbook.</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** 870 µm observations that yielded detections were made during two separate scheduling blocks: 2018 October 18/19 and 2018 October 21. The observations on the first night were done with an eastern array comprising ALMA, APEX, GLT, IRAM30m, and NOEMA. Observations on the second night were made with a western array: ALMA, APEX, GLT, and SMA. The scheduling blocks for both nights are shown along with the one-letter station codes, which are listed in parentheses. All detections are on baselines involving ALMA. The scans that yielded detections on baselines defined by a given station are indicated by the white horizontal ticks centered in each time block: from the top, ticks correspond to XL, XR, YL, and YR mixed polarizations per the legend at upper right. The absence of a tick indicates a nondetection. Three scans at 230 GHz (1.3 mm) were performed at the end of the eastern subarray scheduling block using just the IRAM30m and ALMA facilities.</figcaption></figure>

qualification of such instruments). State-of-the-art instrumentation underpinning the operation of these telescopes, as single-dish facilities and for VLBI, includes cryogenic receivers and wideband digital backends—all refined over many years to optimize performance at millimeter and submillimeter wavelengths. Steady improvements in superconductor–insulator–superconductor junctions have formed the basis for increased bandwidth and sensitivity of millimeter and submillimeter receivers, leading to state-of-the-art systems in use at EHT sites (see Maier et al. 2005, 2012; Tong et al. 2005, 2013; Chenu et al. 2007, 2016; Carter et al. 2012; Mahieu et al. 2012; Kerr et al. 2014; Klein et al. 2014; Belitsky et al. 2018; Han et al. 2018).

Following the successful 1.3 mm VLBI observations in 2017, test observations at 870 µm were conducted on the EHT array in 2018 October. Conditions at the Atacama Large Millimeter/submillimeter Array (ALMA) station during this test, including characterization of the system used there to

phase the array for VLBI, are described in Crew et al. (2023). The present paper describes the VLBI test observations

## 2. Methods

### 2.1. Schedule

The 870 µm fringe test observations consisted of two short scheduling blocks designed for two different subarrays. An eastern subarray, comprising ALMA, the Atacama Pathfinder Experiment (APEX), the Greenland Telescope (GLT), the Institut de Radioastronomie Millimétrique 30 m telescope (IRAM30m), and the Northern Extended Millimeter Array (NOEMA), was scheduled to include blazar sources that were visible in the nighttime hours at all sites: CTA 102, 3C 454.3, and BL Lac. A western subarray, comprising ALMA, APEX, GLT, and the Submillimeter Array (SMA), observed quasars J0423–0120, J0510+1800, J0521+1638, and J0522–3627. The eastern subarray scheduling block was followed by several scans on BL Lac at 1.3 mm wavelength to aid diagnosis in the event of a null result. Schedule blocks for both subarrays were optimized for fringe detection at 870 µm VLBI, and they spanned a duration of between 1 and 2 hr with at least two scans on every source. Most scans lasted 5 minutes.

The observing window consisted of five nights, 2018 October 17–21, between approximately midnight and 2:00 Coordinated Universal Time (UTC) for the eastern subarray scheduling block and between 9:00 and 11:00 UTC for the western subarray scheduling block. Each scheduling block was triggered twice within the observing window. We report herein on successful observations with the eastern array on 2018 October 18–19 and with the western array on 2018 October 21. Details of the scheduling blocks and sources observed are shown in Figure 2.

### 2.2. Instrumentation and Array

Several important technologies developed for 1.3 mm VLBI are leveraged to address the challenges of 870 µm observing, many of which are outlined in Event Horizon Telescope Collaboration et al. (2019b). The VLBI backends, used to condition and digitize signals from the telescope receivers, have a cumulative data rate of 64 Gbps (Vertatschitsch et al. 2015; Tuccari et al. 2017) across four 2 GHz wide bands and two polarizations. Each station is outfitted with a hydrogen maser time standard, which had previously been found to be sufficiently stable for timekeeping in a 1.3 mm VLBI experiment and was expected to be sufficiently stable for 870 µm.

Phased array beamforming capability is implemented at both the SMA (Young et al. 2016) and ALMA (Matthews et al. 2018) array stations. For both these stations, beamformer phasing efficiency at 870 µm, which directly scales the visibility amplitudes measured on baselines to the station, varied from just below 50% to as high as about 80%. These efficiencies are less than what is typical for 1.3 mm (Event Horizon Telescope Collaboration et al. 2019b). Section 3.4 has a discussion relevant to ALMA, SMA, and NOEMA<fnref n="153" /> of phasing efficiency challenges and planned improvements to mitigate these.

<footnote n="153">¹⁵³ NOEMA is also equipped with the phased array, though it was not commissioned at the time of this observation.</footnote>
<figure><img src="image_5.png" /></figure>

**Figure 3.** Scan-averaged and noise-debiased 870 µm fringe amplitude (open blue circles, left axes) and S/N (filled red squares, right axes). Amplitudes and S/N are computed by first dividing each observing scan into short coherently integrated segments, which are then combined incoherently following the procedure in Rogers et al. (1995). Segment length is shown on the horizontal axis. Each subplot shows a different polarization on the ALMA–SMA baseline for a single scan on J0423–0120 (October 21, 09:38 UTC). Other detections listed in Table 1 have a similar dependence on segment duration but generally lower S/N. The noise-debiased amplitude and coherence time were derived using HOPS and are indicated by the horizontal blue dashed line and the vertical solid black line, respectively.

The frequency setup for the 870 µm fringe test is similar to that described in Table 4 of Event Horizon Telescope Collaboration et al. (2019b). Most stations in the array observed a single 2048 MHz band at a 4–6 GHz intermediate frequency (IF) using a 342.6 GHz sky local oscillator (LO).<sup>154</sup> That frequency setup corresponds to a sky frequency range of 346.552–348.6 GHz. Each station observed both circular polarizations, with the exceptions of APEX (right-circular polarization only) and ALMA (dual linear, X and Y). The recorded station data were correlated using DiFX software (Deller et al. 2011) at the MIT Haystack Observatory. Visibility data on baselines to ALMA remained on a mixed-polarization basis (i.e., {X, Y} × {L, R}) because the observing schedules were not long enough to track polarization calibrators over a wide range of parallactic angles, which is necessary for converting the ALMA data from a linear to a circular basis (Martí-Vidal et al. 2016; Matthews et al. 2018; Goddi et al. 2019). Subsequent fringe fitting was done using the Haystack Observatory Postprocessing System (HOPS<sup>155</sup>; Whitney et al. 2004; see also Blackburn et al. 2019).

### 2.2.1. ALMA

ALMA observed in dual linear polarization with IRAM-designed 870 µm (i.e., Band 7) cartridges (Mahieu et al. 2012). The ALMA Phasing System (APS; Matthews et al. 2018) was used to aggregate the collecting area of the active dishes in the ALMA array. The APS capability had been used previously for VLBI science at 3 mm (Issaoun et al. 2019; Okino et al. 2022; Zhao et al. 2022) and 1.3 mm (Event Horizon Telescope Collaboration et al. 2019a, 2019b) but not at shorter

<figure><img src="image_6.png" /></figure>

**Figure 4.** 870 µm contours of incoherently averaged fringe power in 5% increments vs. delay and rate for a single scan on J0423–0120 for the ALMA–SMA baseline (October 21, 09:38 UTC). Other detections reported in Table 1 also exhibit clear peaks vs. delay/rate.

wavelengths, albeit the setup for 870 µm observations is similar to the longer-wavelength bands. In the 870 µm experiment, the four recorded 2.048 GHz subbands were tuned to center frequencies of 335.6, 337.541406, 347.6, and 349.6 GHz. The choice of the 337.541406 GHz frequency results from ALMA-specific tuning restrictions.

The ALMA phased array included 25 12 m antennas during the eastern track and 29 12 m antennas during the western track with a maximum antenna spacing of 600 m in both cases. Wind speeds were greater than 10 m s⁻¹ at the ALMA site. During the eastern track, the phasing efficiency was below 50% for most of the time and at best was about 80%. During the October 21 track (western) in better weather, the phasing efficiency was more stable and greater than approximately 90% (Crew et al. 2023).

### 2.2.2. APEX

The APEX and ALMA stations are colocated, and conditions were similar at the two telescopes. APEX observed using the 345 GHz FLASH+ linear receiver (Klein et al. 2014). That receiver may not have been functioning optimally during the experiment and has since been replaced by the Swedish-ESO PI Instrument for APEX (Belitsky et al. 2018; Meledin et al. 2022). A quarter wave plate was used to achieve circular polarization. Two backends, a ROACH2 Digital Backend (R2DBE; Vertatschitsch et al. 2015) and a Digital BaseBand Converter 3 (Tuccari et al. 2017), were operated in parallel.

### 2.2.3. GLT

The GLT station participated in the observation but at the time was still commissioning specific subsystems. The GLT antenna has operated at Pituffik Space Base, formerly the Thule Airbase site, in Greenland since 2017 August (Inoue et al. 2014; Raffin et al. 2016; Matsushita et al. 2018; Koay et al. 2020; Chen et al. 2023). The GLT observed in dual linear polarization with the IRAM-made 870 µm (i.e., Band 7) cartridges (Mahieu et al. 2012). The 345 GHz receiver on the GLT saw first light in continuum and spectral-line modes in

<footnote n="154"><sup>154</sup> ALMA and SMA used slightly different frequency setups to match the sky frequency of the other stations; see Sections 2.2.1 and 2.2.6.</footnote>

<footnote n="155"><sup>155</sup> https://www.haystack.mit.edu/tech/vlbi/hops.html</footnote>
<table><caption>**Table 1**
870 µm Detections on the Indicated Baselines, Sources, and Polarizations</caption><thead><tr><th>Baseline<sup>a</sup></th><th>Pol.</th><th>Day<sup>b</sup></th><th>Time (hh:ss)</th><th>El. 1 (deg)</th><th>El. 2 (deg)</th><th>|ü - v| (Gλ)</th><th>τ<sub>c</sub> (s)</th><th>Delay (ns)</th><th>Rate (fs s<sup>-1</sup>)</th><th>Amp. (×10<sup>-4</sup>)</th><th>S/N</th></tr></thead><tbody><tr><td colspan="12" style="text-align:center; font-weight:bold;">3C 454.3</td></tr><tr><td>AX</td><td>XR</td><td>292</td><td>00:07</td><td>44.9</td><td>45.0</td><td>0.0026</td><td>8</td><td>4.4</td><td>-1</td><td>0.50</td><td>43.7</td></tr><tr><td>AX</td><td>YR</td><td>292</td><td>00:07</td><td>44.9</td><td>45.0</td><td>0.0026</td><td>8</td><td>5.2</td><td>-1</td><td>0.47</td><td>41.4</td></tr><tr><td colspan="12" style="text-align:center; font-weight:bold;">BL Lac</td></tr><tr><td>AP</td><td>XL</td><td>292</td><td>00:38</td><td>24.6</td><td>42.6</td><td>9.7913</td><td>31</td><td>-4.6</td><td>4</td><td>0.15</td><td>12.2</td></tr><tr><td>AP</td><td>YR</td><td>292</td><td>00:38</td><td>24.6</td><td>42.6</td><td>9.7913</td><td>46</td><td>-8.5</td><td>0</td><td>0.13</td><td>10.8</td></tr><tr><td colspan="12" style="text-align:center; font-weight:bold;">CTA 102</td></tr><tr><td>AP</td><td>YL</td><td>291</td><td>23:52</td><td>49.7</td><td>43.5</td><td>9.9581</td><td>21</td><td>0.9</td><td>-38</td><td>0.18</td><td>13.6</td></tr><tr><td>AX</td><td>XR</td><td>291</td><td>23:44</td><td>48.6</td><td>48.7</td><td>0.0027</td><td>24</td><td>5.6</td><td>-38</td><td>0.23</td><td>19.2</td></tr><tr><td>AX</td><td>XR</td><td>291</td><td>23:52</td><td>49.7</td><td>49.7</td><td>0.0027</td><td>10</td><td>5.2</td><td>-85</td><td>0.23</td><td>20.8</td></tr><tr><td>AX</td><td>YR</td><td>291</td><td>23:44</td><td>48.6</td><td>48.7</td><td>0.0027</td><td>22</td><td>6.3</td><td>-51</td><td>0.21</td><td>17.6</td></tr><tr><td>AX</td><td>YR</td><td>291</td><td>23:52</td><td>49.7</td><td>49.7</td><td>0.0027</td><td>11</td><td>6.0</td><td>-84</td><td>0.22</td><td>18.0</td></tr><tr><td colspan="12" style="text-align:center; font-weight:bold;">J0423-0120</td></tr><tr><td>AS</td><td>XL</td><td>294</td><td>09:22</td><td>48.5</td><td>35.5</td><td>10.8547</td><td>14</td><td>-7.6</td><td>6</td><td>0.54</td><td>47.8</td></tr><tr><td>AS</td><td>XL</td><td>294</td><td>09:30</td><td>46.8</td><td>37.3</td><td>10.8874</td><td>14</td><td>-8.0</td><td>0</td><td>0.70</td><td>62.4</td></tr><tr><td>AS</td><td>XL</td><td>294</td><td>09:38</td><td>45.1</td><td>39.1</td><td>10.9100</td><td>13</td><td>-7.7</td><td>-2</td><td>0.82</td><td>73.1</td></tr><tr><td>AS</td><td>XR</td><td>294</td><td>09:22</td><td>48.5</td><td>35.5</td><td>10.8547</td><td>9</td><td>-7.5</td><td>19</td><td>0.60</td><td>53.4</td></tr><tr><td>AS</td><td>XR</td><td>294</td><td>09:30</td><td>46.8</td><td>37.3</td><td>10.8874</td><td>34</td><td>-7.9</td><td>-0</td><td>0.64</td><td>56.6</td></tr><tr><td>AS</td><td>XR</td><td>294</td><td>09:38</td><td>45.1</td><td>39.1</td><td>10.9100</td><td>9</td><td>-7.5</td><td>-2</td><td>0.79</td><td>70.8</td></tr><tr><td>AS</td><td>YL</td><td>294</td><td>09:22</td><td>48.5</td><td>35.5</td><td>10.8547</td><td>13</td><td>0.8</td><td>19</td><td>0.34</td><td>29.6</td></tr><tr><td>AS</td><td>YL</td><td>294</td><td>09:30</td><td>46.8</td><td>37.3</td><td>10.8874</td><td>17</td><td>0.4</td><td>0</td><td>0.47</td><td>41.3</td></tr><tr><td>AS</td><td>YL</td><td>294</td><td>09:38</td><td>45.1</td><td>39.1</td><td>10.9100</td><td>15</td><td>0.7</td><td>-2</td><td>0.51</td><td>45.2</td></tr><tr><td>AS</td><td>YR</td><td>294</td><td>09:22</td><td>48.5</td><td>35.5</td><td>10.8547</td><td>10</td><td>-5.9</td><td>19</td><td>0.46</td><td>40.7</td></tr><tr><td>AS</td><td>YR</td><td>294</td><td>09:30</td><td>46.8</td><td>37.3</td><td>10.8874</td><td>14</td><td>-6.3</td><td>0</td><td>0.50</td><td>44.2</td></tr><tr><td>AS</td><td>YR</td><td>294</td><td>09:38</td><td>45.1</td><td>39.1</td><td>10.9100</td><td>10</td><td>-5.9</td><td>-3</td><td>0.62</td><td>54.9</td></tr><tr><td>AX</td><td>XR</td><td>294</td><td>09:22</td><td>48.5</td><td>48.5</td><td>0.0028</td><td>27</td><td>-1.0</td><td>-8</td><td>0.14</td><td>12.6</td></tr><tr><td>AX</td><td>XR</td><td>294</td><td>09:30</td><td>46.8</td><td>46.8</td><td>0.0028</td><td>39</td><td>-0.9</td><td>-9</td><td>0.16</td><td>13.0</td></tr><tr><td>AX</td><td>XR</td><td>294</td><td>09:38</td><td>45.1</td><td>45.1</td><td>0.0028</td><td>32</td><td>-0.9</td><td>-11</td><td>0.15</td><td>12.9</td></tr><tr><td>AX</td><td>YR</td><td>294</td><td>09:22</td><td>48.5</td><td>48.5</td><td>0.0028</td><td>30</td><td>0.6</td><td>-7</td><td>0.14</td><td>10.9</td></tr><tr><td>AX</td><td>YR</td><td>294</td><td>09:30</td><td>46.8</td><td>46.8</td><td>0.0028</td><td>29</td><td>0.7</td><td>-9</td><td>0.14</td><td>10.8</td></tr><tr><td colspan="12" style="text-align:center; font-weight:bold;">J0510+1800</td></tr><tr><td>AS</td><td>XL</td><td>294</td><td>10:01</td><td>37.0</td><td>39.6</td><td>10.9218</td><td>30</td><td>-8.0</td><td>-12</td><td>0.10</td><td>8.5</td></tr><tr><td>AS</td><td>XR</td><td>294</td><td>10:01</td><td>37.0</td><td>39.6</td><td>10.9218</td><td>28</td><td>-8.0</td><td>-12</td><td>0.25</td><td>22.3</td></tr><tr><td>AS</td><td>XR</td><td>294</td><td>10:17</td><td>34.5</td><td>43.4</td><td>10.8891</td><td>8</td><td>-8.1</td><td>-0</td><td>0.27</td><td>22.4</td></tr><tr><td>AS</td><td>XR</td><td>294</td><td>10:22</td><td>33.5</td><td>44.8</td><td>10.8682</td><td>22</td><td>2.2</td><td>20</td><td>0.20</td><td>16.6</td></tr><tr><td>AS</td><td>YL</td><td>294</td><td>10:01</td><td>37.0</td><td>39.6</td><td>10.9218</td><td>10</td><td>0.3</td><td>-12</td><td>0.20</td><td>18.1</td></tr><tr><td>AS</td><td>YL</td><td>294</td><td>10:17</td><td>34.5</td><td>43.4</td><td>10.8891</td><td>23</td><td>0.2</td><td>11</td><td>0.25</td><td>21.3</td></tr><tr><td>AS</td><td>YL</td><td>294</td><td>10:22</td><td>33.5</td><td>44.8</td><td>10.8682</td><td>29</td><td>-6.6</td><td>2</td><td>0.17</td><td>14.2</td></tr><tr><td>AS</td><td>YR</td><td>294</td><td>10:01</td><td>37.0</td><td>39.6</td><td>10.9218</td><td>28</td><td>-6.3</td><td>-14</td><td>0.12</td><td>10.1</td></tr><tr><td>AS</td><td>YR</td><td>294</td><td>10:17</td><td>34.5</td><td>43.4</td><td>10.8891</td><td>6<sup>c</sup></td><td>-6.5</td><td>0</td><td>0.14</td><td>11.5</td></tr><tr><td>AS</td><td>YR</td><td>294</td><td>10:22</td><td>33.5</td><td>44.8</td><td>10.8682</td><td>10<sup>c</sup></td><td>3.8</td><td>81</td><td>0.11</td><td>9.7</td></tr><tr><td colspan="12" style="text-align:center; font-weight:bold;">J0522-3627</td></tr><tr><td>AS</td><td>XR</td><td>294</td><td>10:37</td><td>53.0</td><td>18.0</td><td>10.3188</td><td>12<sup>c</sup></td><td>-4.7</td><td>38</td><td>0.12</td><td>10.1</td></tr><tr><td>AS</td><td>XR</td><td>294</td><td>10:45</td><td>51.4</td><td>19.2</td><td>10.4084</td><td>24</td><td>-4.9</td><td>8</td><td>0.20</td><td>12.1</td></tr><tr><td>AS</td><td>YL</td><td>294</td><td>10:37</td><td>53.0</td><td>18.0</td><td>10.3188</td><td>29</td><td>3.5</td><td>-4</td><td>0.12</td><td>10.3</td></tr><tr><td>AS</td><td>YL</td><td>294</td><td>10:45</td><td>51.4</td><td>19.2</td><td>10.4084</td><td>22</td><td>3.4</td><td>-4</td><td>0.16</td><td>14.1</td></tr><tr><td>AX</td><td>XR</td><td>294</td><td>10:37</td><td>53.0</td><td>52.9</td><td>0.0030</td><td>31</td><td>0.8</td><td>-1</td><td>0.31</td><td>26.9</td></tr><tr><td>AX</td><td>XR</td><td>294</td><td>10:45</td><td>51.4</td><td>51.4</td><td>0.0030</td><td>39</td><td>0.8</td><td>25</td><td>0.25</td><td>15.3</td></tr><tr><td>AX</td><td>YR</td><td>294</td><td>10:37</td><td>53.0</td><td>52.9</td><td>0.0030</td><td>31</td><td>2.3</td><td>1</td><td>0.31</td><td>27.0</td></tr><tr><td>AX</td><td>YR</td><td>294</td><td>10:45</td><td>51.4</td><td>51.4</td><td>0.0030</td><td>31</td><td>2.4</td><td>25</td><td>0.29</td><td>24.6</td></tr></tbody></table>

**Notes.**

<footnote><sup>a</sup> Baselines: AX (ALMA–APEX), AP (ALMA–IRAM30m), AS (ALMA–SMA).</footnote>

<footnote><sup>b</sup> Day of year in 2018.</footnote>

<footnote><sup>c</sup> The S/N was insufficient to fit the coherence time. The reported value is the segmentation time that achieves the greatest S/N for the scan.</footnote>
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Detections on various targets at 345 GHz (see Table 1). The *u*-*v* locations of 230 GHz detections on M87*+* during the EHT 2017 April campaign are shown in gray including low-S/N scans at (25 µas)⁻¹.</figcaption></figure>

2018 August. Pointing and focus calibration at 345 GHz were still in the commissioning phase during the 870 µm observation reported here. The GLT pointing system has since been fully commissioned for recent and future VLBI observing. Similarly, final adjustments to the dish surface had yet to be made, and the surface accuracy was estimated to be 170 µm rms during the observations reported here. Subsequent improvements have led to rms surface accuracy in the 17–40 µm range (see Table 7 in Chen et al. 2023).

### 2.2.4. IRAM30m

The IRAM30m telescope used the heterodyne Eight MIxer Receiver (Carter et al. 2012) in the 870 µm band also known as E330. The setup and preobserving checks were analogous to a regular Global Millimeter VLBI Array or EHT session. The opacity at 870 µm during the scheduled VLBI observations was high and would not typically have triggered single-dish science operation at this wavelength.

### 2.2.5. NOEMA

Portions of the NOEMA station were still being commissioned during the 870 µm experiment. NOEMA observed in dual polarization as a single-antenna station, not as a phased array. The NOEMA receiver was a dual-polarization single-sideband unit (Chenu et al. 2016) with a 4 GHz bandpass. Recording was with a 16 Gbps R2DBE. The NOEMA phased array has since been commissioned for VLBI observing.

### 2.2.6. SMA

The SMA station observed with seven antennas arranged in the compact configuration with a maximum baseline of 69.1 m. The SMA Wideband Astronomical ROACH2 Machine (SWARM; Primiani et al. 2016; Young et al. 2016) was run with the VLBI beamformer mode activated, producing a coherent phased array sum of the seven antennas formatted for VLBI recording. As expected, the phasing efficiency was lower

<figure><img src="image_8.png" /><figcaption>**Figure 6.** 1.3 mm amplitude (open blue circles, left axes) and S/N (filled red squares, right axes) vs. the duration of coherently integrated segments, which are incoherently averaged. Each subplot shows a different polarization on the baseline between ALMA and IRAM30m for a single scan on BL Lac on October 19, 01:13 UTC. Other BL Lac detections listed in Table 2 have a similar dependence on segment duration. The noise-debiased amplitude and coherence time were derived using HOPS and are indicated by the horizontal blue dashed line and the vertical solid black line, respectively. These data were calibrated in the same manner as the 870 µm detections.</figcaption></figure>

than for 1.3 mm operations. The sky LO was set to 341.6 GHz, not 342.6 GHz, to match the SWARM sky coverage with the other stations, compensating for a different IF to baseband LO because SWARM uses its own block downconverter rather than the standard EHT single-dish equipment. The data were recorded in the frequency domain at the standard SMA clock rate (4.576 Gbps), which differs from the standard EHT single-dish sample rate of 4.096 Gbps (Vertatschitsch et al. 2015). Adaptive Phased Array Interpolating Downsampler for SWARM (APHIDS) postprocessing was completed to interpolate and invert (from frequency domain to time domain) the SWARM data sets in preparation for VLBI correlation. After APHIDS processing, the SMA EHT data product matches that produced by the standard SMA single-dish station in sample rate and is also a time series matching the standard EHT single-dish data product.

## 3. Results and Discussion

Figure 1 shows that the conditions during the experiment were mixed across the array. While the observatories do not measure 870 µm (345 GHz) opacity directly, we use MERRA-2 reanalysis and radiative transfer (Paine 2022) that is validated by measurements at 225 GHz (Figure 1, black lines) to estimate $\tau_{345}$. For the eastern subarray on October 18/19, $\tau_{345}$ was 0.2 at the ALMA and APEX sites and 0.8 at IRAM30m. For the western subarray on October 21, $\tau_{345}$ was approximately 0.17 at the ALMA and APEX sites and 0.7 at SMA. During the experiment, the opacities at GLT and NOEMA were unfavorable, and detections on baselines to those stations were not achieved; however, both stations have weather that is compatible with 870 µm observing and will likely yield high-frequency detections in the future (see, e.g., Raymond et al. 2021; Matsushita et al. 2022). Atmospheric conditions can change rapidly: $\tau_{225}$ at the SMA decreased by nearly a factor of 4 in the hours following the experiment.
<figure><img src="image_9.png" /><figcaption>**Figure 7.** 1.3 mm contours of incoherently averaged fringe power in 5% increments vs. delay and rate for the baseline between ALMA and IRAM30m. This example is for a single scan on BL Lac taken on October 19, 01:13 UTC. Other detections reported in Table 2 also exhibit clear peaks vs. in delay-delay rate search space.</figcaption></figure>

### 3.1. 870 µm (345 GHz) Fringes

In VLBI, recorded data from all sites are brought to a central processing facility where data streams from each pair of sites are cross-correlated. The resulting complex correlation quantities provide a dimensionless measure of the electric field coherence between the two sites, which is proportional to a Fourier component of the brightness distribution of the target source. The correlation processor uses an a priori model to align the site data streams, recreating the exact geometry of the physical baseline connecting the two sites at the time of observation. Because the a priori model is imperfect, after processing, the cross-correlation phase typically varies as a function of time and frequency due to residual delay and delay rate, respectively. To average the correlation signal over frequency and time, the correlator output is thus searched over a range of delay and delay rate to find a peak in correlator power—a process also known as “fringe fitting” (Thompson et al. 2017). In this experiment, the correlator output was searched by dividing each scan into short segments and incoherently averaging them. The incoherent averaging technique (Rogers et al. 1995) estimates noise-debiased VLBI quantities, and it is well suited to processing low signal-to-noise ratio (S/N) VLBI data on sparse arrays as it allows integration beyond the nominal atmospheric coherence time. Figure 3 shows the dependence of amplitude in units of $10^4$ and S/N on the duration of the segments for a sample scan on source J0423–0120 for the baseline comprising the ALMA and SMA stations. All four cross-hand polarizations are plotted. The scan identifier 294–0938 in Figure 3 corresponds to the day UTC for the beginning of the scan, where the day is the number of days since 2018 January 1 (294 is October 21) and UTC is the scan start time. The noise-debiased amplitude (Rogers et al. 1995) in Figure 3 is indicated by the horizontal blue dashed line. As the segment duration decreases, the effect of decoherence is reduced, so the S/N increases.

Compared to a single coherent integration over a full scan (approximately 300 s in most of the measurements),

<figure><img src="image_10.png" /><figcaption>**Figure 8.** Allan deviation for 870 µm (345 GHz) scans observed on the ALMA–SMA baseline (blue lines). For comparison, red lines show the Allan deviation for high-S/N scans (nominally 5 minutes long) during the 1.3 mm (230 GHz) 2017 EHT campaign (Event Horizon Telescope Collaboration et al. 2019b). Weather variability during the 2017 campaign is responsible for the spread in those scans. The means of the individual Allan deviation traces are shown in bold for the two frequencies. The 870 µm and 1.3 mm mean traces approach the nominal Allan deviation for a pair of T4 Science brand iMaser 3000 model masers (Thompson et al. 2017) at short timescales. At intermediate timescales, atmospheric turbulence can become important. The Allan deviation associated with Kolmogorov turbulence is plotted for a set of nominal parameters (Treuhaft & Lanyi 1987).</figcaption></figure>

<table><caption>**Table 2**

1.3 mm Detections on the ALMA–IRAM30m Baseline toward BL Lac for Indicated Polarizations</caption><thead><tr><th>Elevation<br>(ALMA/<br>IRAM30m)<br>(deg)</th><th>Baseline<br>Length<br>(Gλ)</th><th>τ<sub>c</sub><br>(s)</th><th>Delay<br>(ns)</th><th>Rate<br>(fs s<sup>-1</sup>)</th><th>Amp.</th><th>S/N</th></tr></thead><tbody><tr><td colspan="7" style="text-align: center; font-weight: bold;">XL</td></tr><tr><td>24.5/38.3</td><td>6.4327</td><td>5</td><td>-5.3</td><td>-98</td><td>1.66</td><td>134.0</td></tr><tr><td>24.4/37.3</td><td>6.4422</td><td>7</td><td>-5.3</td><td>-66</td><td>1.49</td><td>120.0</td></tr><tr><td>24.3/36.6</td><td>6.4476</td><td>32</td><td>-5.3</td><td>-14</td><td>1.47</td><td>187.3</td></tr><tr><td colspan="7" style="text-align: center; font-weight: bold;">YR</td></tr><tr><td>24.5/38.3</td><td>6.4327</td><td>6</td><td>-0.8</td><td>-99</td><td>1.77</td><td>143.0</td></tr><tr><td>24.4/37.3</td><td>6.4422</td><td>7</td><td>-0.8</td><td>-66</td><td>1.52</td><td>122.4</td></tr><tr><td>24.3/36.6</td><td>6.4476</td><td>32</td><td>-0.8</td><td>-14</td><td>1.49</td><td>189.8</td></tr><tr><td colspan="7" style="text-align: center; font-weight: bold;">XR</td></tr><tr><td>24.5/38.3</td><td>6.4327</td><td>6</td><td>-0.7</td><td>-98</td><td>1.56</td><td>125.4</td></tr><tr><td>24.4/37.3</td><td>6.4422</td><td>7</td><td>-0.7</td><td>-66</td><td>1.37</td><td>110.4</td></tr><tr><td>24.3/36.6</td><td>6.4476</td><td>32</td><td>-0.7</td><td>-13</td><td>1.38</td><td>176.1</td></tr><tr><td colspan="7" style="text-align: center; font-weight: bold;">YL</td></tr><tr><td>24.5/38.3</td><td>6.4327</td><td>6</td><td>-5.4</td><td>-98</td><td>1.42</td><td>114.4</td></tr><tr><td>24.4/37.3</td><td>6.4422</td><td>7</td><td>-5.4</td><td>-66</td><td>1.24</td><td>100.1</td></tr><tr><td>24.3/36.6</td><td>6.4476</td><td>32</td><td>-5.4</td><td>-14</td><td>1.21</td><td>153.5</td></tr></tbody></table>

**Note.** Scans listed top to bottom on October 19 begin at 01:03, 01:09, and 01:13 UTC.

incoherently averaging the parts of a segmented scan increases the S/N by up to a factor of 2 on many of the measurements, yielding higher confidence in the detections. For most of the measurements, S/N values asymptote at the shortest segment
durations. Ordinarily, we would expect the S/N values to
decrease as the segments are shortened below the coherence
time. The behavior we observe could be indicative of a
changing coherence during the scan consistent with the windy
conditions at ALMA (Crew et al. 2023).

Contours of fringe power versus multiband delay and rate
are plotted in Figure 4 for a single scan of J0423–0120 on the
ALMA–SMA baseline. The measurement exhibits a definitive
peak in fringe power for each of the cross-hand polarizations.
The rates are all centered near 0. Multiband delays fall within
an ambiguity search window of (−8.53 ns, 8.53 ns) as they are
derived from measurements spaced at ALMA’s channel
separation of 58.592375 MHz (Matthews et al. 2018; Event
Horizon Telescope Collaboration et al. 2019d).

The fringe detection threshold was conservatively set at S/N > 7 to prevent false detections, and all resulting detections are summarized in Table 1 ordered by target source. The maximum spatial frequencies sampled are greater than 10.9 Gλ between ALMA and the SMA, which significantly exceeds the largest spatial frequencies sampled by the EHT for M87* at 1.3 mm on the longest baseline between Hawaii and Europe (approximately 8 Gλ). The highest-S/N detections exceed 70. Simultaneous detections in all four polarization products were achieved on the ALMA–SMA baseline for J0423–0120. The zero-baseline flux densities at 870 µm were obtained from the ALMA local interferometry (Crew et al. 2023). The flux densities were 1.4, 1.0, 2.4, 1.2, and 4.9 Jy on CTA 102, BL Lac, J0423–0120, J0510+1800, and J0522–3627, respectively. The source structure of the targets in this work is not known a priori, so it is not possible to say with precision how the correlated amplitudes should vary as a function of baseline length. Furthermore, these observations were designed to be a detection experiment and not carried out with all procedures that would allow robust VLBI flux density calibration. Nevertheless, the S/N on the ALMA–APEX baselines appears to be anomalously low given the short baseline length, which would ordinarily be sensitive to both small-scale structure (10–100 µas) and larger-scale structure (10–100 mas). This is likely attributable to phase instabilities suspected in the APEX receiver (see Section 2.2.2), which has since been retired. Follow-on experiments, already scheduled, will focus on calibration and robust flux density measurements versus baseline length.

HOPS reports two coherence times: one corresponding to the
point below which there is only a small amount of coherence
loss within the uncertainty of amplitudes and another
corresponding to the maximum S/N. For most of the scans in
Table 1, we report the former. In a few low-S/N cases where
the routine was unable to fit the coherence, the coherence time
based on S/N is reported instead. The coherence times across
baselines range from approximately 10 to 30 s for most cases.
For BL Lac, the longer coherence times may be an artifact of
the moderate S/N.

3.2. 1.3 mm (230 GHz) Comparison

Presently, the EHT observes at 1.3 mm (Event Horizon
Telescope Collaboration et al. 2019b). Figure 5 compares the
Fourier components of the 870 µm detections on various
sources to the 1.3 mm coverage of the 2017 EHT array on
M87* (Event Horizon Telescope Collaboration et al. 2019d).
The 870 µm detections on ALMA–IRAM30m and ALMA–
SMA baselines have a higher nominal angular resolution

(19 µas) than the highest-resolution M87* detections (nomin-
ally 25 µas).

For a source-specific comparison of the 1.3 mm and 870 µm
bands, ALMA and IRAM30m observed BL Lac at 1.3 mm
during three scans at the end of the eastern subarray scheduling
block of the 2018 October session. Those data were searched
using the same HOPS incoherent averaging method as was
used for the 870 µm observations and provide an independent
application of the approach. The 1.3 mm scans provide a check
of the 870 µm processing and a point of comparison for the
870 µm detections.

The amplitude and S/N values for one of the 1.3 mm scans
are plotted in Figure 6 versus the duration of incoherently
averaged segments. The S/N values are approximately tenfold
greater at 1.3 mm than at 870 µm (see Figure 3), which likely
results from a combination of factors that boost sensitivity at
the longer wavelength: lower opacity, lower receiver noise,
greater aperture efficiency, a wider beam, greater coherence,
and greater source flux density. The coherence time determined
using HOPS was comparable for the three scans to what was
found at 870 µm: on the order of 6–30 s. As with the 870 µm
measurements, the S/N values asymptote as the segment
duration decreases below the coherence time. The consistency
of the S/N trends in the 870 µm and 1.3 mm scans suggests
that the behavior is a real feature of the data and not an artifact
of the analysis.

Comparison of the 1.3 mm and 870 µm wavelengths
observing BL Lac also shows that the latter is a much more
difficult regime in which to operate. The atmospheric
conditions at the IRAM30m site (see Figure 1; τ<sub>345</sub> ~ 0.8)
were not ideal for 870 µm observing during the test. At
1.3 mm, strong detections were obtained on all polarizations for
each of the three attempted scans. At 870 µm, detections were
made on just two of four polarizations for a single ALMA–
IRAM30m scan, and none were made on other BL Lac
baselines. The tenfold greater S/N values at 1.3 mm are
consistent with the system equivalent flux density (SEFD). The
SEFDs on BL Lac scans at ALMA were approximately 150 Jy
at 1.3 mm versus 580 Jy at 870 µm (a factor of 3.9 change). At
IRAM30m, SEFDs during the BL Lac scans were 3800 Jy at
1.3 mm versus 10<sup>5</sup> Jy at 870 µm (a factor of approximately 25
change). The S/N is inversely proportional to the root product
of the SEFDs, or √(3.9 × 25) ≈ 10, which explains the
behavior across observing wavelengths. The significantly
greater noise at 870 µm as well as the other losses associated
with narrower beamwidth or coherence is the likely reason for
nondetections to some stations and on certain scans.

Fringe power contours at 1.3 mm are plotted as a function of
multiband delay and rate in Figure 7, exhibiting obvious peaks.
The delays for each of the four polarization cross products is
consistent across scans, and the 1.3 mm fringes are summarized
in Table 2. All four polarization cross-hands are detected in
each of the three 1.3 mm scans. The 6.4 Gλ spatial frequencies
are 50% smaller than the 870 µm scans on the AP baseline,
which corresponds to the frequency scaling between the two
bands. The 1.3 mm zero-baseline flux density of BL Lac
deduced from the ALMA local interferometry (Crew et al.
2023) was 1.2 Jy.

3.3. Coherence and Allan Deviation

It is convenient to characterize the phase noise of an
interferometer by its Allan deviation, which is a measure of
<figure><img src="image_11.png" /><figcaption>**Figure 9.** Left: visibility amplitudes for simulated observations of M87\* (top) and Sgr A\* (bottom) at observing wavelengths of 1.3 mm (gray) and 0.87 mm (red). The synthetic data have been generated using the ngehtsim package assuming array specifications appropriate for the phase 2 ngEHT array from Doeleman et al. (2023), including simultaneous dual-band observations, the use of the FPT calibration technique, and 16 GHz of bandwidth at both frequencies. Data points are colored by their S/N on an integration time of 5 minutes, and data points with S/N < 3 have been flagged. Right: images produced from GRMHD simulations of the M87\* (top two panels; Event Horizon Telescope Collaboration et al. 2019e) and Sgr A\* (bottom two panels; Event Horizon Telescope Collaboration et al. 2022b) accretion flows, used to generate the synthetic data shown in the left panels. Both simulations have been ray-traced at observing wavelengths of 1.3 mm (gray) and 0.87 mm (red), and the frequency-dependent effects of interstellar scattering have been applied to the Sgr A\* images (Johnson 2016; Johnson et al. 2018).</figcaption></figure>

fractional stability for an oscillator, time standard, or any time-
variable process. When computing the Allan deviation of an
observed VLBI interferometer phase, one normalizes by the
frequency of observation to produce a dimensionless quantity.
The relationships of Allan deviation to the statistical variance,
coherence, and phase power spectrum can be found in
Thompson et al. (2017). Examples of the Allan deviation of
VLBI systems referenced to hydrogen maser time standards
and operating at 1.3 cm and 3 mm wavelengths can be found in
Rogers & Moran (1981) and Rogers et al. (1984), respectively,
and show that at short wavelengths, decoherence is a potential
concern. Alternatives to hydrogen masers for short-wavelength
VLBI work have been explored (e.g., Doeleman et al. 2011). In
this section, we compare the observed Allan deviation of the
VLBI interferometric phase to limiting factors including the
stability of time and frequency standards used in the

experiment as well as instabilities due to atmospheric
turbulence.

Figure 8 shows the Allan deviation for 870 µm scans on the ALMA–SMA baseline. Over most integration times, the 870 µm Allan deviation is comparable to but greater than the maser–maser reference. The 870 µm traces exhibit relatively small scan-to-scan variation during the course of the brief fringe test when conditions were relatively stable. For comparison, Figure 8 also shows the Allan deviations for a large number of high-S/N 1.3 mm scans from the 2017 EHT campaign (Event Horizon Telescope Collaboration et al. 2019d). At times of less than about 5 s, the red 1.3 mm traces all approach the limit set by the maser references. At times longer than 5 s, the red traces are noticeably scattered. The scatter exists because of the variability of atmospheric conditions during the course of an observing campaign.
The tropospheric delay is essentially independent of wavelength for wavelengths longer than about 600 µm as described by the Smith-Weintraub equation (see Thompson et al. 2017, Chapter 13). Thus, the Allan deviation is expected to be independent of wavelength for our observations. When the atmospheric conditions are stable, the 1.3 mm Allan deviation for individual scans approaches the maser-maser limit across all integration times. The mean of the 1.3 mm scans is within a factor of approximately 2 of the mean of the 870 µm traces. The 870 µm mean Allan deviation on the plot happens to be lower than the 1.3 mm mean for most integration times. However, we do not consider this difference to be significant given the extremely small 870 µm data set. Further, the observations in 2017 April and 2018 October were of course made in differing weather conditions.

To assess the impact of atmospheric turbulence at longer times, the Allan deviation associated with atmospheric Kolmogorov turbulence is plotted for a set of nominal conditions following the approach outlined by Treuhaft & Lanyi (1987): 10 m s⁻¹ wind speed, 2 km troposphere scale height, 1.99 × 10⁻⁷ m⁻¹/³ Kolmogorov coefficient, and independent distant sites. The nominal Kolmogorov trace exceeds the maser-maser Allan deviation at longer times, where we expect atmospheric effects to dominate. Beyond 10 s, the nominal Kolmogorov trace matches the shape of the 1.3 mm mean. Although the 870 µm mean falls somewhere between the maser-maser and nominal Kolmogorov limits, the atmospheric contribution may become more apparent in the future with scans spanning more variable weather conditions.

### 3.4. Phasing Efficiency

An important figure of merit when used to monitor the performance of phased array beamformers is phasing efficiency. This is a measure of how effectively outputs of the dishes in the local array are coherently summed to synthesize a single IF output from the array's aggregated collecting area. For each array site, periodic estimates of phasing efficiency over time are stored with other essential metadata for use in calibration.

The ALMA and SMA phased arrays experienced lower and more variable phasing efficiency during the 870 µm test than is typical for 1.3 mm observing in similar conditions. At 870 µm, atmospheric opacity is between 3 and 3.5 times that for 1.3 mm given the same precipitable water vapor. Further source fluxes decline with increasing frequency or shorter wavelength. Both of these factors result in a lower local array fringe S/N. There is thus greater error in the fits of the antenna phase corrections. Tuning within the band avoids the deep absorption lines due to atmospheric water resonances at 325 and 385 GHz, which would reduce the S/N still further. Also, the atmospheric phase fluctuations tracked by the adaptive phased array system have a greater amplitude for observations in the higher-frequency band. Crew et al. (2023) note that that moist, windy conditions tend to diminish phasing efficiency, and the winds were quite high at ALMA during the test. At dry, less windy times, ALMA obtained higher phasing efficiencies approaching 100%. While NOEMA participated in this test with a single dish, not as a phased array, all of these factors are expected to apply as well to NOEMA, which is now equipped with a phased array backend capable of beamforming in both the 1.3 mm and 870 µm bands.

Water vapor radiometer (WVR) based phasing corrections were not in use during the 2018 test. Independent testing at ALMA shows that fast WVR corrections are effective at improving the efficiency when phase fluctuations are primarily due to water vapor. Phasing control loop algorithms are constantly being improved and in future will be better tuned to the 870 µm wave band. These improvements will expand the opportunities for 870 µm observing in a wider range of weather conditions and on weaker sources. Despite these challenges, VLBI detections at 870 µm can be readily achieved even when phasing efficiencies are relatively low and in nonideal weather conditions.

## 4. Future Directions

Achieving 870 µm VLBI fringes has strong implications for science directions that future global arrays operating at this wavelength can explore. As angular resolution scales with wavelength, we anticipate improving resolution from ~23 to ~15 µas on the longest EHT baselines (Figure 5). Plasma propagation processes typically scale as wavelength squared, so at 870 µm, scatter broadening of Sgr A* reduces to ~5 µas, further sharpening resolution and increasing signal-to-noise on the longest VLBI baselines. Similarly, Faraday rotation measured across the bandpass of EHT receivers at 870 µm can be used to improve estimates of accretion plasma densities and magnetic field geometries close to EHT targets. For both Sgr A* and M87*, the images at 870 µm and 1.3 mm are determined predominantly by the achromatic gravitational lensing and hence should exhibit similar characteristics, implying that the aggregate Fourier coverage of VLBI observations at different frequencies can be used to improve modeling of the gravitationally lensed emission and the imaging fidelity generally (Chael et al. 2023). Figure 9 shows Fourier amplitudes as a function of radius for GRMHD<fnref n="156" /> models of M87* and Sgr A*. Inclusion of 345 GHz observations adds coverage in the visibility plane regions not sampled at 230 GHz, and it extends baseline lengths for higher angular resolution as well as enhanced overall sampling of Fourier spatial frequencies to allow dynamical reconstructions of accretion and jet launch close to the event horizon.

There are several developments that will increase the sensitivity and flexibility of 870 µm VLBI in the near future. Next-generation VLBI backends (Doeleman et al. 2023) will allow an increase in data capture rates from 64 to 128 Gb s⁻¹ (per observing frequency band), lowering detection thresholds by $\sqrt{2}$. Additional use of the frequency phase transfer (FPT) technique (Rioja et al. 2023) through simultaneous observations at 86, 230, and 345 GHz will extend coherent integration times at higher frequencies, further increasing sensitivity. In optimal cases, this increase will be the square root of the ratio of coherence times at 86 GHz and 345 GHz ($\sqrt{\tau_c(86)/\tau_c(345)}$). And the participation of more telescopes at high-altitude sites will make the EHT array more robust against adverse weather conditions, increasing the opportunities for staging 870 µm VLBI observations (Raymond et al. 2021; Doeleman et al. 2023). Anticipated upgrades to ALMA will be exceptionally useful to advance 870 µm VLBI and are planned on a similar timeline (~2030) as the ngEHT upgrade (Carpenter et al. 2023). In particular, the projected doubling of the continuum bandwidth of ALMA will match the ngEHT specifications, and

<footnote n="156">¹⁵⁶ General relativistic magnetohydrodynamic.</footnote>
a subarray capability at ALMA will enable simultaneous multiband observations that benefit from FPT as noted above. In sum, the prospects for routine 870 µm VLBI in the near future are excellent.

## 5. Conclusions

VLBI fringe detections on baselines between ALMA–APEX, ALMA–IRAM30m, and ALMA–SMA have been achieved at 870 µm for multiple AGN sources. S/Ns were between approximately 10 and 70. Despite marginal weather conditions across the array, detections to multiple stations and sources were obtained. This work demonstrates that the EHT instrumentation is viable at 870 µm (345 GHz) and will provide a critical advance in array capability. EHT-wide observations at 870 µm would yield a fringe spacing of about 15 µas and, with a full track of coverage, would significantly enhance the fine details of the EHT images of AGN and horizon-scale targets (Doeleman et al. 2019, 2023; Johnson et al. 2023).

## Acknowledgments

This work was supported by the National Science Foundation (grants AST-1935980, AST-1743747, AST-1440254), an ALMA Cycle 5 North America Development Award, the Gordon and Betty Moore Foundation (awards GBMF3561, GBMF5278, and GBMF10423), and a generous gift from the Deepak Raghavan Family Foundation. Work on this project was conducted in part at the Black Hole Initiative at Harvard University (funded through grants 60477, 61479, and 62286 from the John Templeton Foundation and grant GBMF8273 from the Gordon and Betty Moore Foundation). This work is partly based on observations carried out with the IRAM 30 m telescope and the NOEMA Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). The IRAM NOEMA phasing project was supported by the European Research Council (ERC) Synergy Grant “Black-HoleCam: Imaging the Event Horizon of Black Holes” (grant 610058). The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. The SMA gratefully acknowledges the efforts of its staff for supporting these observations, including those of the operator on duty, R. Howie. This publication is based on data acquired with the APEX. APEX is a collaboration between the Max-Planck-Institut für Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory. This work was an activity external to JPL, and effort by A.R. was not in their capacity as an employee of the Jet Propulsion Laboratory, California Institute of Technology. The GLT is supported by the Ministry of Science and Technology (MOST) of Taiwan (103-2119-M-001-010-MY2, 105-2119-M-001-042, 106-2119-M-001-013, 107-2119-M-001-041, 108-2112-M-001-048, 109-2124-M-001-005, 110-2124-M-001-007) and the National Science and Technology Council (NSTC) of Taiwan (111-2124-M-001-005, 112-2124-M-001-014).

The Event Horizon Telescope Collaboration additionally thanks the following organizations and programs: the Academia Sinica; the Academy of Finland (projects 274477, 284495, 312496, 315721); the Agencia Nacional de Investigación y Desarrollo (ANID), Chile via NCN19058 (TITANs), Fondecyt 1221421 and BASAL FB210003; the Alexander von Humboldt

Stiftung; an Alfred P. Sloan Research Fellowship; Allegro, the European ALMA Regional Centre node in the Netherlands, the NL astronomy research network NOVA, and the astronomy institutes of the University of Amsterdam, Leiden University, and Radboud University; the ALMA North America Development Fund; the Astrophysics and High Energy Physics program by MCIN (with funding from European Union NextGenerationEU, PRTR-C17I1); the Brinson Foundation; “la Caixa” Foundation (ID 100010434) through fellowship codes LCF/BQ/DI22/11940027 and LCF/BQ/DI22/11940030; Chandra DD7-18089X and TM6-17006X; the China Scholarship Council; the China Postdoctoral Science Foundation fellowships (2020M671266, 2022M712084); Consejo Nacional de Humanidades, Ciencia y Tecnología (CON-AHCYT, Mexico, projects U0004-246083, U0004-259839, F0003-272050, M0037-279006, F0003-281692, 104497, 275201, 263356); the Colfuturo Scholarship; the Consejería de Economía, Conocimiento, Empresas y Universidad of the Junta de Andalucía (grant P18-FR-1769); the Consejo Superior de Investigaciones Científicas (grant 2019AEP112); the Delaney Family via the Delaney Family John A. Wheeler Chair at Perimeter Institute; Dirección General de Asuntos del Personal Académico-Universidad Nacional Autónoma de México (DGAPA-UNAM, projects IN112820 and IN108324); the Dutch Research Council (NWO) for the VICI award (grant 639.043.513), the grant OCENW.KLEIN.113, and the Dutch Black Hole Consortium (with project No. NWA 1292.19.202) of the research program the National Science Agenda; the Dutch National Supercomputers, Cartesius and Snellius (NWO grant 2021.013); the EACOA Fellowship awarded by the East Asia Core Observatories Association, which consists of the Academia Sinica Institute of Astronomy and Astrophysics, the National Astronomical Observatory of Japan, Center for Astronomical Mega-Science, Chinese Academy of Sciences, and the Korea Astronomy and Space Science Institute; the European Union Horizon 2020 research and innovation program under grant agreements RadioNet (No. 730562), M2FINDERS (No. 101018682), and FunFiCO (No. 777740); the European Research Council for the advanced grant “JETSET: Launching, propagation and emission of relativistic jets from binary mergers and across mass scales” (grant No. 884631); the European Horizon Europe staff exchange (SE) program HORIZON-MSCA-2021-SE-01 grant NewFunFiCO (No. 10108625); the Horizon ERC Grants 2021 program under grant agreement No. 101040021; the FAPESP (Fundação de Amparo á Pesquisa do Estado de São Paulo) under grant 2021/01183-8; the Fondo CAS-ANID folio CAS220010; the Generalitat Valenciana (grants APOSTD/2018/177 and ASFAE/2022/018) and GenT Program (project CIDEGENT/2018/021); the Institute for Advanced Study; 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; DFG research grant “Jet physics on horizon scales and beyond” (grant No. 443220636); Joint Columbia/Flatiron Postdoctoral Fellowship (research at the Flatiron Institute is supported by the Simons Foundation); the Japan Ministry of Education, Culture, Sports, Science and Technology (MEXT; grant JPMXP1020200109); the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for JSPS Research Fellowship (JP17J08829); the Joint Institute for Computational Fundamental Science, Japan; the Key Research
Program of Frontier Sciences, Chinese Academy of Sciences (CAS; grants QYZDJ-SSW-SLH057, QYZDJSSW-SYS008, ZDBS-LY-SLH011); the Leverhulme Trust Early Career Research Fellowship; the Max-Planck-Gesellschaft (MPG); the Max Planck Partner Group of the MPG and the CAS; the MEXT/JSPS KAKENHI (grants 18KK0090, JP21H01137, JP18H03721, JP18K13594, 18K03709, JP19K14761, 18H01245, 25120007, 19H01943, 21H01137, 21H04488, 22H00157, 23K03453); the MICINN Research Projects PID2019-108995GB-C22, PID2022-140888NB-C22; 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, 106-2923-M-001-005, 107-2119-M-001-017, 107-2119-M-001-020, 107-2119-M-110-005, 107-2923-M-001-009, 108-2112-M-001-051, 108-2923-M-001-002, 109-2112-M-001-025, 109-2923-M-001-001, 110-2112-M-001-033, 110-2923-M-001-001, and 112-2112-M-003-010-MY3); the Ministry of Education (MoE) of Taiwan Yushan Young Scholar Program; the Physics Division, National Center for Theoretical Sciences of Taiwan; the National Aeronautics and Space Administration (NASA; Fermi Guest Investigator grant 80NSSC23K1508, NASA Astrophysics Theory Program grant 80NSSC20K0527, NASA NuSTAR award 80NSSC20K0645); NASA Hubble Fellowship grants HST-HF2-51431.001-A and HST-HF2-51482.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555; the National Institute of Natural Sciences (NINS) of Japan; the National Key Research and Development Program of China (grants 2016YFA0400704, 2017YFA0402703, 2016YFA0400702); the National Science and Technology Council (NSTC; grants NSTC 111-2112-M-001-041, NSTC 111-2124-M-001-005, NSTC 112-2124-M-001-014); the US National Science Foundation (NSF; grants AST-0096454, AST-0352953, AST-0521233, AST-0705062, AST-0905844, AST-0922984, AST-1126433, OIA-1126433, AST-1140030, DGE-1144085, AST-1207704, AST-1207730, AST-1207752, MRI-1228509, OPP-1248097, AST-1310896, AST-1555365, AST-1614868, AST-1615796, AST-1715061, AST-1716327, AST-1726637, OISE-1743747, AST-1816420, AST-1952099, AST-2034306, AST-2205908, AST-2307887); NSF Astronomy and Astrophysics Postdoctoral Fellowship (AST-1903847); the Natural Science Foundation of China (grants 11650110427, 10625314, 11721303, 11725312, 11873028, 11933007, 11991052, 11991053, 12192220, 12192223, 12273022, 12325302, 12303021); 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 Research Foundation of Korea (the Global PhD Fellowship Grant: grants NRF-2015H1A2A1033752, the Korea Research Fellowship Program: NRF-2015H1D3A1066561, Brain Pool Program: 2019H1D3A1A01102564, Basic Research Support grant 2019R1F1A1059721, 2021R1A6A3A0108 6420, 2022R1C1C1005255, 2022R1F1A1075115); Netherlands Research School for Astronomy (NOVA) Virtual Institute of Accretion (VIA) postdoctoral fellowships; NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation; 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 Portuguese Foundation for Science and Technology (FCT) grants (Individual CEEC program—5th edition, https://doi.org/10.54499/UIDP/04106/2020, PTDC/FIS-AST/3041/2020, CERN/FIS-PAR/0024/2021, 2022.045 60.PTDC); the Princeton Gravity Initiative; the Spanish Ministerio de Ciencia e Innovación (grants PGC2018-098915-B-C21, AYA2016-80889-P, PID2019-108995GB-C21, PID2020-1174 04GB-C21); the University of Pretoria for financial aid in the provision of the new Cluster Server nodes and SuperMicro (USA) for a SEEDING GRANT approved toward these nodes in 2020; the Shanghai Municipality orientation program of basic research for international scientists (grant No. 22JC1410600); the Shanghai Pilot Program for Basic Research, Chinese Academy of Science, Shanghai Branch (JCYJ-SHFY-2021-013); the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709); the Spanish Ministry for Science and Innovation grant CEX2021-001131-S funded by MCIN/AEI/10.13039/501100011033; the Spinoza Prize SPI 78-409; the South African Research Chairs Initiative through the South African Radio Astronomy Observatory (SARAO; grant ID 77948), which is a facility of the National Research Foundation (NRF), an agency of the Department of Science and Innovation (DSI) of South Africa; the Swedish Research Council (VR); the Taplin Fellowship; the Toray Science Foundation; the UK Science and Technology Facilities Council (grant no. ST/X508329/1); 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); and the YCAA Prize Postdoctoral Fellowship.

We thank the staff 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#2011.0.00010.E, and ADS/JAO.ALMA#2011.0.00012.E. See https://almascience.nrao.edu/alma-data/eht-2018 for more detail and access to the data. 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. This research used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, which is supported by the Office of Science of the U.S. Department of Energy under contract No. DE-AC05-00OR22725; the ASTROVIVES FEDER infrastructure, with project code IDIFEDER-2021-086; and the computing cluster of the Shanghai VLBI correlator supported by the Special Fund for Astronomy from the Ministry of Finance in China. We also thank the Center for Computational Astrophysics, National Astronomical Observatory of Japan. This work was
supported by FAPESP (Fundação de Amparo a Pesquisa do Estado de São Paulo) under grant 2021/01183-8.

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 staff of the ALMA, 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 EHT station is located, has for the indigenous Hawaiian people.

## ORCID iDs

Alexander W. Raymond https://orcid.org/0000-0002-5779-4767

Sheperd S. Doeleman https://orcid.org/0000-0002-9031-0904

Keiichi Asada https://orcid.org/0000-0001-6988-8763

Lindy Blackburn https://orcid.org/0000-0002-9030-642X

Geoffrey C. Bower https://orcid.org/0000-0003-4056-9982

Michael Bremer https://orcid.org/0000-0001-7511-3745

Dominique Broguiere https://orcid.org/0000-0001-9151-6683

Ming-Tang Chen https://orcid.org/0000-0001-6573-3318

Geoffrey B. Crew https://orcid.org/0000-0002-2079-3189

Sven Dornbusch https://orcid.org/0000-0003-3211-3352

Vincent L. Fish https://orcid.org/0000-0002-7128-9345

Roberto García https://orcid.org/0000-0002-6584-7443

Olivier Gentaz https://orcid.org/0000-0002-0115-4605

Ciriaco Goddi https://orcid.org/0000-0002-2542-7743

Michael H. Hecht https://orcid.org/0000-0002-4114-4583

Yau-De Huang https://orcid.org/0000-0001-8783-6211

Michael Janssen https://orcid.org/0000-0001-8685-6544

Garrett K. Keating https://orcid.org/0000-0002-3490-146X

Jun Yi Koay https://orcid.org/0000-0002-7029-6658

Thomas P. Krichbaum https://orcid.org/0000-0002-4892-9586

Wen-Ping Lo https://orcid.org/0000-0003-1869-2503

Satoki Matsushita https://orcid.org/0000-0002-2127-7880

Lynn D. Matthews https://orcid.org/0000-0002-3728-8082

James M. Moran https://orcid.org/0000-0002-3882-4414

Nimesh Patel https://orcid.org/0000-0002-6021-9421

Dominic W. Pesce https://orcid.org/0000-0002-5278-9221

Venkatessh Ramakrishnan https://orcid.org/0000-0002-9248-086X

Alan L. Roy https://orcid.org/0000-0002-1931-0135

Salvador Sánchez https://orcid.org/0000-0002-8042-5951

Remo P. J. Tilanus https://orcid.org/0000-0002-6514-553X

Michael Titus https://orcid.org/0000-0001-9001-3275

Pablo Torne https://orcid.org/0000-0001-8700-6058

Jan Wagner https://orcid.org/0000-0003-1105-6109

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Sergio A. Dzib https://orcid.org/0000-0001-6010-6200

Ralph P. Eatough https://orcid.org/0000-0001-6196-4135

Razieh Emami https://orcid.org/0000-0002-2791-5011

Heino Falcke https://orcid.org/0000-0002-2526-6724
Joseph Farah https://orcid.org/0000-0003-4914-5625

Edward Fomalont https://orcid.org/0000-0002-9036-2747

H. Alyson Ford https://orcid.org/0000-0002-9797-0972

Marianna Foschi https://orcid.org/0000-0001-8147-4993

Raquel Fraga-Encinas https://orcid.org/0000-0002-5222-1361

Per Friberg https://orcid.org/0000-0002-8010-8454

Christian M. Fromm https://orcid.org/0000-0002-1827-1656

Antonio Fuentes https://orcid.org/0000-0002-8773-4933

Peter Galison https://orcid.org/0000-0002-6429-3872

Charles F. Gammie https://orcid.org/0000-0001-7451-8935

Boris Georgiev https://orcid.org/0000-0002-3586-6424

Roman Gold https://orcid.org/0000-0003-2492-1966

Arturo I. Gómez-Ruiz https://orcid.org/0000-0001-9395-1670

José L. Gómez https://orcid.org/0000-0003-4190-7613

Minfeng Gu (顧敏峰) https://orcid.org/0000-0002-4455-6946

Mark Gurwell https://orcid.org/0000-0003-0685-3621

Kazuhiro Hada https://orcid.org/0000-0001-6906-772X

Daryl Haggard https://orcid.org/0000-0001-6803-2138

Ronald Hesper https://orcid.org/0000-0003-1918-6098

Dirk Heumann https://orcid.org/0000-0002-7671-0047

Luis C. Ho (何子山) https://orcid.org/0000-0001-6947-5846

Paul Ho https://orcid.org/0000-0002-3412-4306

Mareki Honma https://orcid.org/0000-0003-4058-9000

Chih-Wei L. Huang https://orcid.org/0000-0001-5641-3953

Lei Huang (黃磊) https://orcid.org/0000-0002-1923-227X

Shiro Ikeda https://orcid.org/0000-0002-2462-1448

C. M. Violette Impellizzeri https://orcid.org/0000-0002-3443-2472

Makoto Inoue https://orcid.org/0000-0001-5037-3989

Sara Issaoun https://orcid.org/0000-0002-5297-921X

David J. James https://orcid.org/0000-0001-5160-4486

Buell T. Jannuzi https://orcid.org/0000-0002-1578-6582

Britton Jeter https://orcid.org/0000-0003-2847-1712

Wu Jiang (江悟) https://orcid.org/0000-0001-7369-3539

Alejandra Jiménez-Rosales https://orcid.org/0000-0002-2662-3754

Michael D. Johnson https://orcid.org/0000-0002-4120-3029

Svetlana Jorstad https://orcid.org/0000-0001-6158-1708

Abhishek V. Joshi https://orcid.org/0000-0002-2514-5965

Taehyun Jung https://orcid.org/0000-0001-7003-8643

Ramesh Karuppusamy https://orcid.org/0000-0002-5307-2919

Tomohisa Kawashima https://orcid.org/0000-0001-8527-0496

Mark Kettenis https://orcid.org/0000-0002-6156-5617

Dong-Jin Kim https://orcid.org/0000-0002-7038-2118

Jae-Young Kim https://orcid.org/0000-0001-8229-7183

Jongsoo Kim https://orcid.org/0000-0002-1229-0426

Junhan Kim https://orcid.org/0000-0002-4274-9373

Motoki Kino https://orcid.org/0000-0002-2709-7338

Prashant Kocherlakota https://orcid.org/0000-0001-7386-7439

Patrick M. Koch https://orcid.org/0000-0003-2777-5861

Shoko Koyama https://orcid.org/0000-0002-3723-3372

Carsten Kramer https://orcid.org/0000-0002-4908-4925

Joana A. Kramer https://orcid.org/0009-0003-3011-0454

Michael Kramer https://orcid.org/0000-0002-4175-2271

Cheng-Yu Kuo https://orcid.org/0000-0001-6211-5581

Noemi La Bella https://orcid.org/0000-0002-8116-9427

Sang-Sung Lee https://orcid.org/0000-0002-6269-594X

Aviad Levis https://orcid.org/0000-0001-7307-632X

Zhiyuan Li (李志远) https://orcid.org/0000-0003-0355-6437

Rocco Lico https://orcid.org/0000-0001-7361-2460

Greg Lindahl https://orcid.org/0000-0002-6100-4772

Michael Lindqvist https://orcid.org/0000-0002-3669-0715

Mikhail Lisakov https://orcid.org/0000-0001-6088-3819

Jun Liu (刘俊) https://orcid.org/0000-0002-7615-7499

Kuo Liu https://orcid.org/0000-0002-2953-7376

Elisabetta Liuzzo https://orcid.org/0000-0003-0995-5201

Andrei P. Lobanov https://orcid.org/0000-0003-1622-1484

Laurent Loinard https://orcid.org/0000-0002-5635-3345

Colin J. Lonsdale https://orcid.org/0000-0003-4062-4654

Amy E. Lowitz https://orcid.org/0000-0002-4747-4276

Ru-Sen Lu (路如森) https://orcid.org/0000-0002-7692-7967

Nicholas R. MacDonald https://orcid.org/0000-0002-6684-8691

Jirong Mao (毛基荣) https://orcid.org/0000-0002-7077-7195

Nicola Marchili https://orcid.org/0000-0002-5523-7588

Sera Markoff https://orcid.org/0000-0001-9564-0876

Daniel P. Marrone https://orcid.org/0000-0002-2367-1080

Alan P. Marscher https://orcid.org/0000-0001-7396-3332

Iván Martí-Vidal https://orcid.org/0000-0003-3708-9611

Lia Medeiros https://orcid.org/0000-0003-2342-6728

Karl M. Tenen https://orcid.org/0000-0001-6459-0669

Izumi Mizuno https://orcid.org/0000-0002-7210-6264

Yosuke Mizuno https://orcid.org/0000-0002-8131-6730

Joshua Montgomery https://orcid.org/0000-0003-0345-8386

Kotaro Moriyama https://orcid.org/0000-0003-1364-3761

Monika Moscibrodzka https://orcid.org/0000-0002-4661-6332

Wanga Mulaudzi https://orcid.org/0000-0003-4514-625X

Cornelia Müller https://orcid.org/0000-0002-2739-2994

Hendrik Müller https://orcid.org/0000-0002-9250-0197

Alejandro Mus https://orcid.org/0000-0003-0329-6874

Gibwa Musoke https://orcid.org/0000-0003-1984-189X

Ioannis Myserlis https://orcid.org/0000-0003-3025-9497

Hiroshi Nagai https://orcid.org/0000-0003-0292-3645

Neil M. Nagar https://orcid.org/0000-0001-6920-662X

Masanori Nakamura https://orcid.org/0000-0001-6081-2420

Gopal Narayanan https://orcid.org/0000-0002-4723-6569

Iniyan Natarajan https://orcid.org/0000-0001-8242-4373

Antonios Nathanail https://orcid.org/0000-0002-1655-9912

Joey Neilsen https://orcid.org/0000-0002-8247-786X

Chunchong Ni https://orcid.org/0000-0003-1361-5699

Michael A. Nowak https://orcid.org/0000-0001-6923-1315

Junghwan Oh https://orcid.org/0000-0002-4991-9638

Hiroki Okino https://orcid.org/0000-0003-3779-2016

Héctor Raúl Olivares Sánchez https://orcid.org/0000-0001-6833-7580

Tomoaki Oyama https://orcid.org/0000-0003-4046-2923

Feryal Özel https://orcid.org/0000-0003-4413-1523

Daniel C. M. Palumbo https://orcid.org/0000-0002-7179-3816
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Vincent Piétu https://orcid.org/0009-0006-3497-397X

Oliver Porth https://orcid.org/0000-0002-4584-2557

Ben Prather https://orcid.org/0000-0002-0393-7734

Giacomo Principe https://orcid.org/0000-0003-0406-7387

Dimitrios Psaltis https://orcid.org/0000-0003-1035-3240

Hung-Yi Pu https://orcid.org/0000-0001-9270-8812

Ramprasad Rao https://orcid.org/0000-0002-1407-7944

Mark G. Rawlings https://orcid.org/0000-0002-6529-202X

Angelo Ricarte https://orcid.org/0000-0001-5287-0452

Bart Ripperda https://orcid.org/0000-0002-7301-3908

Freek Roelofs https://orcid.org/0000-0001-5461-3687

Cristina Romero-Cañizales https://orcid.org/0000-0001-6301-9073

Eduardo Ros https://orcid.org/0000-0001-9503-4892

Arash Roshanineshat https://orcid.org/0000-0002-8280-9238

Ignacio Ruiz https://orcid.org/0000-0002-0965-5463

Chet Ruszczyk https://orcid.org/0000-0001-7278-9707

Kazi L. J. Rygl https://orcid.org/0000-0003-4146-9043

David Sánchez-Argüelles https://orcid.org/0000-0002-7344-9920

Miguel Sánchez-Portal https://orcid.org/0000-0003-0981-9664

Mahito Sasada https://orcid.org/0000-0001-5946-9960

Kaushik Satapathy https://orcid.org/0000-0003-0433-3585

Tuomas Savolainen https://orcid.org/0000-0001-6214-1085

Jonathan Schonfeld https://orcid.org/0000-0002-8909-2401

Karl-Friedrich Schuster https://orcid.org/0000-0003-2890-9454

Lijing Shao https://orcid.org/0000-0002-1334-8853

Zhiqiang Shen (沈志强) https://orcid.org/0000-0003-3540-8746

Des Small https://orcid.org/0000-0003-3723-5404

Bong Won Sohn https://orcid.org/0000-0002-4148-8378

Jason SooHoo https://orcid.org/0000-0003-1938-0720

León David Sosapanta Salas https://orcid.org/0000-0003-1979-6363

Kamal Souccar https://orcid.org/0000-0001-7915-5272

Joshua S. Stanway https://orcid.org/0009-0003-7659-4642

He Sun (孙赫) https://orcid.org/0000-0003-1526-6787

Fumie Tazaki https://orcid.org/0000-0003-0236-0600

Alexandra J. Tetarenko https://orcid.org/0000-0003-3906-4354

Paul Tiede https://orcid.org/0000-0003-3826-5648

Kenji Toma https://orcid.org/0000-0002-7114-6010

Teresa Toscano https://orcid.org/0000-0003-3658-7862

Efthalia Traianou https://orcid.org/0000-0002-1209-6500

Sascha Trippe https://orcid.org/0000-0003-0465-1559

Matthew Turk https://orcid.org/0000-0002-5294-0198

Ilse van Bemmel https://orcid.org/0000-0001-5473-2950

Huib Jan van Langevelde https://orcid.org/0000-0002-0230-5946

Daniel R. van Rossum https://orcid.org/0000-0001-7772-6131

Jesse Vos https://orcid.org/0000-0003-3349-7394

Derek Ward-Thompson https://orcid.org/0000-0003-1140-2761

John Wardle https://orcid.org/0000-0002-8960-2942

Jasmin E. Washington https://orcid.org/0000-0002-7046-0470

Robert Wharton https://orcid.org/0000-0002-7416-5209

Kaj Wiik https://orcid.org/0000-0002-0862-3398

Gunther Witzel https://orcid.org/0000-0003-2618-797X

Michael F. Wondrak https://orcid.org/0000-0002-6894-1072

George N. Wong https://orcid.org/0000-0001-6952-2147

Qingwen Wu (吴庆文) https://orcid.org/0000-0003-4773-4987

Nitika Yadlapalli https://orcid.org/0000-0003-3255-4617

Paul Yamaguchi https://orcid.org/0000-0002-6017-8199

Aristomenis Yfantis https://orcid.org/0000-0002-3244-7072

Doosoo Yoon https://orcid.org/0000-0001-8694-8166

Ziri Younsi https://orcid.org/0000-0001-9283-1191

Wei Yu (于威) https://orcid.org/0000-0002-5168-6052

Feng Yuan (袁峰) https://orcid.org/0000-0003-3564-6437

Ye-Fei Yuan (袁业飞) https://orcid.org/0000-0002-7330-4756

J. Anton Zensus https://orcid.org/0000-0001-7470-3321

Shuo Zhang https://orcid.org/0000-0002-2967-790X

Guang-Yao Zhao https://orcid.org/0000-0002-4417-1659

Shan-Shan Zhao (赵杉杉) https://orcid.org/0000-0002-9774-3606

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