Polarimetric properties of Event Horizon Telescope targets from ALMA

Ciriaco Goddi

0000-0002-2542-7743]Ciriaco Goddi
Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands
Leiden Observatory—Allegro, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands

0000-0003-3708-9611]Iván Martí-Vidal
Departament d'Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, E-46100 Burjassot, València, Spain
Observatori Astronòmic, Universitat de València, C. Catedrático José Beltrán 2, E-46980 Paterna, València, Spain

0000-0002-2985-7994]Hugo Messias
Joint ALMA Observatory, Alonso de Cordova 3107, Vitacura 763-0355, Santiago de Chile, Chile

0000-0003-4056-9982]Geoffrey C. Bower
Institute of Astronomy and Astrophysics, Academia Sinica, 645 N. A'ohoku Place, Hilo, HI 96720, USA

0000-0002-3351-760X]Avery E. Broderick
Perimeter Institute for Theoretical Physics, 31 Caroline Street North, Waterloo, ON, N2L 2Y5, Canada
Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada
Waterloo Centre for Astrophysics, University of Waterloo, Waterloo, ON N2L 3G1 Canada

0000-0003-3903-0373]Jason Dexter
JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309, USA

0000-0002-2367-1080]Daniel P. Marrone
Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA

0000-0002-4661-6332]Monika Moscibrodzka
Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands

0000-0003-0292-3645]Hiroshi Nagai
National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
Department of Astronomical Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

0000-0001-6993-1696]Juan Carlos Algaba
Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia

Keiichi Asada
Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.

0000-0002-2079-3189]Geoffrey B. Crew
Massachusetts Institute of Technology Haystack Observatory, 99 Millstone Road, Westford, MA 01886, USA

0000-0003-4190-7613]José L. Gómez
Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain

C. M. Violette Impellizzeri
Leiden Observatory—Allegro, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands
Joint ALMA Observatory, Alonso de Cordova 3107, Vitacura 763-0355, Santiago de Chile, Chile

0000-0001-8685-6544]Michael Janssen
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

Matthias Kadler
Institut für Theoretische Physik und Astrophysik Universität Würzburg, Emil Fischer, Str. 31 97074, Würzburg, Germany

0000-0002-4892-9586]Thomas P. Krichbaum
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0001-7361-2460]Rocco Lico
Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0002-3728-8082]Lynn D. Matthews
Massachusetts Institute of Technology Haystack Observatory, 99 Millstone Road, Westford, MA 01886, USA

Antonios Nathanail
Institut für Theoretische Physik, Goethe-Universität Frankfurt, Max-von-Laue-Straße 1, D-60438 Frankfurt am Main, Germany
Department of Physics, National and Kapodistrian University of Athens, Panepistimiopolis, GR 15783 Zografos, Greece

0000-0001-5287-0452]Angelo Ricarte
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

0000-0001-9503-4892]Eduardo Ros
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0001-9283-1191]Ziri Younsi
Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
Institut für Theoretische Physik, Goethe-Universität Frankfurt, Max-von-Laue-Straße 1, D-60438 Frankfurt am Main, Germany

0000-0002-9475-4254]Kazunori Akiyama
Massachusetts Institute of Technology Haystack Observatory, 99 Millstone Road, Westford, MA 01886, USA
National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA

0000-0002-9371-1033]Antxon Alberdi
Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain

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

0000-0003-3457-7660]Richard Anantua
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA

0000-0002-2200-5393]Rebecca Azulay
Departament d'Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, E-46100 Burjassot, València, Spain
Observatori Astronòmic, Universitat de València, C. Catedrático José Beltrán 2, E-46980 Paterna, València, Spain
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0003-3090-3975]Anne-Kathrin Baczko
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

David Ball
Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA

0000-0003-0476-6647]Mislav Baloković
Yale Center for Astronomy & Astrophysics, 52 Hillhouse Avenue, New Haven, CT 06511, USA
Department of Physics, Yale University, P.O. Box 2018120, New Haven, CT 06520, USA

0000-0002-9290-0764]John Barrett
Massachusetts Institute of Technology Haystack Observatory, 99 Millstone Road, Westford, MA 01886, USA

0000-0002-5108-6823]Bradford A. Benson
Fermi National Accelerator Laboratory, MS209, P.O. Box 500, Batavia, IL, 60510, USA
Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA

Dan Bintley
East Asian Observatory, 660 N. A'ohoku Place, Hilo, HI 96720, USA

0000-0002-9030-642X]Lindy Blackburn
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

0000-0002-5929-5857]Raymond Blundell
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

Wilfred Boland
Nederlandse Onderzoekschool voor Astronomie (NOVA), PO Box 9513, 2300 RA Leiden, The Netherlands

0000-0003-0077-4367]Katherine L. Bouman
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA

0000-0002-6530-5783]Hope Boyce
Department of Physics, McGill University, 3600 rue University, Montréal, QC H3A 2T8, Canada
McGill Space Institute, McGill University, 3550 rue University, Montréal, QC H3A 2A7, Canada

Michael Bremer
Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint Martin d'Hères, France

0000-0002-2322-0749]Christiaan D. Brinkerink
Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands

0000-0002-2556-0894]Roger Brissenden
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

0000-0001-9240-6734]Silke Britzen
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

Dominique Broguiere
Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint Martin d'Hères, France

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

0000-0003-1157-4109]Do-Young Byun
Korea Astronomy and Space Science Institute, Daedeok-daero 776, Yuseong-gu, Daejeon 34055, Republic of Korea
University of Science and Technology, Gajeong-ro 217, Yuseong-gu, Daejeon 34113, Republic of Korea

John E. Carlstrom
Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL, 60637, USA
Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
Department of Physics, University of Chicago, 5720 South Ellis Avenue, Chicago, IL 60637, USA
Enrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA

0000-0003-2966-6220]Andrew Chael
Princeton Center for Theoretical Science, Jadwin Hall, Princeton University, Princeton, NJ 08544, USA

0000-0001-6337-6126]Chi-kwan Chan
Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
Data Science Institute, University of Arizona, 1230 N. Cherry Ave., Tucson, AZ 85721, USA

0000-0002-2878-1502]Shami Chatterjee
Cornell Center for Astrophysics and Planetary Science, Cornell University, Ithaca, NY 14853, USA

0000-0002-2825-3590]Koushik Chatterjee
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands

Ming-Tang Chen
Institute of Astronomy and Astrophysics, Academia Sinica, 645 N. A'ohoku Place, Hilo, HI 96720, USA

Yongjun Chen (陈永军)
Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, People's Republic of China
Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Nanjing 210008, People's Republic of China

0000-0001-6327-8462]Paul M. Chesler
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA

0000-0001-6083-7521]Ilje Cho
Korea Astronomy and Space Science Institute, Daedeok-daero 776, Yuseong-gu, Daejeon 34055, Republic of Korea
University of Science and Technology, Gajeong-ro 217, Yuseong-gu, Daejeon 34113, Republic of Korea

0000-0001-6820-9941]Pierre Christian
Physics Department, Fairfield University, 1073 North Benson Road, Fairfield, CT 06824, USA

0000-0003-2448-9181]John E. Conway
Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, SE-43992 Onsala, Sweden

James M. Cordes
Cornell Center for Astrophysics and Planetary Science, Cornell University, Ithaca, NY 14853, USA

0000-0001-9000-5013]Thomas M. Crawford
Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL, 60637, USA

0000-0002-3945-6342]Alejandro Cruz-Osorio
Institut für Theoretische Physik, Goethe-Universität Frankfurt, Max-von-Laue-Straße 1, D-60438 Frankfurt am Main, Germany

0000-0001-6311-4345]Yuzhu Cui
Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Mizusawa, Oshu, Iwate 023-0861, Japan
Department of Astronomical Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

0000-0002-2685-2434]Jordy Davelaar
Department of Astronomy and Columbia Astrophysics Laboratory, Columbia University, 550 W 120th Street, New York, NY 10027, USA
Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands

0000-0002-9945-682X]Mariafelicia De Laurentis
Dipartimento di Fisica “E. Pancini”, Universitá di Napoli “Federico II”, Compl. Univ. di Monte S. Angelo, Edificio G, Via Cinthia, I-80126, Napoli, Italy
Institut für Theoretische Physik, Goethe-Universität Frankfurt, Max-von-Laue-Straße 1, D-60438 Frankfurt am Main, Germany
INFN Sez. di Napoli, Compl. Univ. di Monte S. Angelo, Edificio G, Via Cinthia, I-80126, Napoli, Italy

0000-0003-1027-5043]Roger Deane
Wits Centre for Astrophysics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg 2050, South Africa
Department of Physics, University of Pretoria, Hatfield, Pretoria 0028, South Africa
Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, Makhanda 6140, South Africa

0000-0003-1269-9667]Jessica Dempsey
East Asian Observatory, 660 N. A'ohoku Place, Hilo, HI 96720, USA

0000-0003-3922-4055]Gregory Desvignes
LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France

0000-0002-9031-0904]Sheperd S. Doeleman
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

0000-0001-6196-4135]Ralph P. Eatough
National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100101, PR China
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0002-2526-6724]Heino Falcke
Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands

0000-0003-4914-5625]Joseph Farah
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
University of Massachusetts Boston, 100 William T. Morrissey Boulevard, Boston, MA 02125, USA

0000-0002-7128-9345]Vincent L. Fish
Massachusetts Institute of Technology Haystack Observatory, 99 Millstone Road, Westford, MA 01886, USA

Ed Fomalont
National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA

0000-0002-9797-0972]H. Alyson Ford
Steward Observatory and Department of Astronomy, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA

0000-0002-5222-1361]Raquel Fraga-Encinas
Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands

William T. Freeman
Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 32-D476, 77 Massachusetts Ave., Cambridge, MA 02142, USA
Google Research, 355 Main St., Cambridge, MA 02142, USA

Per Friberg
East Asian Observatory, 660 N. A'ohoku Place, Hilo, HI 96720, USA

Christian M. Fromm
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
Institut für Theoretische Physik, Goethe-Universität Frankfurt, Max-von-Laue-Straße 1, D-60438 Frankfurt am Main, Germany

0000-0002-8773-4933]Antonio Fuentes
Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain

0000-0002-6429-3872]Peter Galison
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Department of History of Science, Harvard University, Cambridge, MA 02138, USA
Department of Physics, Harvard University, Cambridge, MA 02138, USA

0000-0001-7451-8935]Charles F. Gammie
Department of Physics, University of Illinois, 1110 West Green Street, Urbana, IL 61801, USA
Department of Astronomy, University of Illinois at Urbana-Champaign, 1002 West Green Street, Urbana, IL 61801, USA

0000-0002-6584-7443]Roberto García
Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint Martin d'Hères, France

Olivier Gentaz
Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint Martin d'Hères, France

0000-0002-3586-6424]Boris Georgiev
Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada
Waterloo Centre for Astrophysics, University of Waterloo, Waterloo, ON N2L 3G1 Canada

0000-0003-2492-1966]Roman Gold
CP3-Origins, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark
Perimeter Institute for Theoretical Physics, 31 Caroline Street North, Waterloo, ON, N2L 2Y5, Canada

0000-0001-9395-1670]Arturo I. Gómez-Ruiz
Instituto Nacional de Astrofísica, Óptica y Electrónica. Apartado Postal 51 y 216, 72000. Puebla Pue., México
Consejo Nacional de Ciencia y Tecnología, Av. Insurgentes Sur 1582, 03940, Ciudad de México, México

0000-0002-4455-6946]Minfeng Gu (顾敏峰)
Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, People's Republic of China
Key Laboratory for Research in Galaxies and Cosmology, Chinese Academy of Sciences, Shanghai 200030, People's Republic of China

0000-0003-0685-3621]Mark Gurwell
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

0000-0001-6906-772X]Kazuhiro Hada
Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Mizusawa, Oshu, Iwate 023-0861, Japan
Department of Astronomical Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

0000-0001-6803-2138]Daryl Haggard
Department of Physics, McGill University, 3600 rue University, Montréal, QC H3A 2T8, Canada
McGill Space Institute, McGill University, 3550 rue University, Montréal, QC H3A 2A7, Canada

Michael H. Hecht
Massachusetts Institute of Technology Haystack Observatory, 99 Millstone Road, Westford, MA 01886, USA

0000-0003-1918-6098]Ronald Hesper
NOVA Sub-mm Instrumentation Group, Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands

0000-0001-6947-5846]Luis C. Ho (何子山)
Department of Astronomy, School of Physics, Peking University, Beijing 100871, People's Republic of China
Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People's Republic of China

Paul Ho
Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.

0000-0003-4058-9000]Mareki Honma
Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Mizusawa, Oshu, Iwate 023-0861, Japan
Department of Astronomical Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

0000-0001-5641-3953]Chih-Wei L. Huang
Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.

Lei Huang (黄磊)
Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, People's Republic of China
Key Laboratory for Research in Galaxies and Cosmology, Chinese Academy of Sciences, Shanghai 200030, People's Republic of China

David H. Hughes
Instituto Nacional de Astrofísica, Óptica y Electrónica. Apartado Postal 51 y 216, 72000. Puebla Pue., México

Makoto Inoue
Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.

0000-0002-5297-921X]Sara Issaoun
Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands

0000-0001-5160-4486]David J. James
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

Buell T. Jannuzi
Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA

0000-0003-2847-1712]Britton Jeter
Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada
Waterloo Centre for Astrophysics, University of Waterloo, Waterloo, ON N2L 3G1 Canada

0000-0001-7369-3539]Wu Jiang (江悟)
Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, People's Republic of China

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

0000-0002-4120-3029]Michael D. Johnson
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

0000-0001-6158-1708]Svetlana Jorstad
Institute for Astrophysical Research, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA
Astronomical Institute, St. Petersburg University, Universitetskij pr., 28, Petrodvorets,198504 St.Petersburg, Russia

0000-0001-7003-8643]Taehyun Jung
Korea Astronomy and Space Science Institute, Daedeok-daero 776, Yuseong-gu, Daejeon 34055, Republic of Korea
University of Science and Technology, Gajeong-ro 217, Yuseong-gu, Daejeon 34113, Republic of Korea

0000-0001-7387-9333]Mansour Karami
Perimeter Institute for Theoretical Physics, 31 Caroline Street North, Waterloo, ON, N2L 2Y5, Canada
Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada

0000-0002-5307-2919]Ramesh Karuppusamy
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0001-8527-0496]Tomohisa Kawashima
Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8582, Japan

0000-0002-3490-146X]Garrett K. Keating
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

0000-0002-6156-5617]Mark Kettenis
Joint Institute for VLBI ERIC (JIVE), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands

0000-0002-7038-2118]Dong-Jin Kim
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0001-8229-7183]Jae-Young Kim
Korea Astronomy and Space Science Institute, Daedeok-daero 776, Yuseong-gu, Daejeon 34055, Republic of Korea
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

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

0000-0002-4274-9373]Junhan Kim
Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA

0000-0002-2709-7338]Motoki Kino
National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
Kogakuin University of Technology & Engineering, Academic Support Center, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan

0000-0002-7029-6658]Jun Yi Koay
Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.

Yutaro Kofuji
Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Mizusawa, Oshu, Iwate 023-0861, Japan
Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

0000-0003-2777-5861]Patrick M. Koch
Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.

0000-0002-3723-3372]Shoko Koyama
Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.

0000-0002-4175-2271]Michael Kramer
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0002-4908-4925]Carsten Kramer
Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint Martin d'Hères, France

Cheng-Yu Kuo
Physics Department, National Sun Yat-Sen University, No. 70, Lien-Hai Rd, Kaosiung City 80424, Taiwan, R.O.C
Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.

0000-0003-3234-7247]Tod R. Lauer
National Optical Astronomy Observatory, 950 North Cherry Ave., Tucson, AZ 85719, USA

0000-0002-6269-594X]Sang-Sung Lee
Korea Astronomy and Space Science Institute, Daedeok-daero 776, Yuseong-gu, Daejeon 34055, Republic of Korea

0000-0001-7307-632X]Aviad Levis
California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA

0000-0001-5841-9179]Yan-Rong Li (李彦荣)
Key Laboratory for Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, 19B Yuquan Road, Shijingshan District, Beijing, People's Republic of China

0000-0003-0355-6437]Zhiyuan Li (李志远)
School of Astronomy and Space Science, Nanjing University, Nanjing 210023, People's Republic of China
Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Nanjing 210023, People's Republic of China

0000-0002-3669-0715]Michael Lindqvist
Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, SE-43992 Onsala, Sweden

0000-0002-6100-4772]Greg Lindahl
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

0000-0002-7615-7499]Jun Liu (刘俊)
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0002-2953-7376]Kuo Liu
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0003-0995-5201]Elisabetta Liuzzo
Italian ALMA Regional Centre, INAF-Istituto di Radioastronomia, Via P. Gobetti 101, I-40129 Bologna, Italy

Wen-Ping Lo
Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.
Department of Physics, National Taiwan University, No.1, Sect.4, Roosevelt Rd., Taipei 10617, Taiwan, R.O.C

Andrei P. Lobanov
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0002-5635-3345]Laurent Loinard
Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Morelia 58089, México
Instituto de Astronomía, Universidad Nacional Autónoma de México, CdMx 04510, México

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

0000-0002-7692-7967]Ru-Sen Lu (路如森)
Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, People's Republic of China
Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Nanjing 210008, People's Republic of China
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0002-6684-8691]Nicholas R. MacDonald
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0002-7077-7195]Jirong Mao (毛基荣)
Yunnan Observatories, Chinese Academy of Sciences, 650011 Kunming, Yunnan Province, People's Republic of China
Center for Astronomical Mega-Science, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing, 100012, People's Republic of China
Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, 650011 Kunming, People's Republic of China

0000-0002-5523-7588]Nicola Marchili
Italian ALMA Regional Centre, INAF-Istituto di Radioastronomia, Via P. Gobetti 101, I-40129 Bologna, Italy
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0001-9564-0876]Sera Markoff
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
Gravitation Astroparticle Physics Amsterdam (GRAPPA) Institute, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

0000-0001-7396-3332]Alan P. Marscher
Institute for Astrophysical Research, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA

0000-0002-2127-7880]Satoki Matsushita
Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.

0000-0003-2342-6728]Lia Medeiros
School of Natural Sciences, Institute for Advanced Study, 1 Einstein Drive, Princeton, NJ 08540, USA
Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA

0000-0001-6459-0669]Karl M. Menten
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0002-7210-6264]Izumi Mizuno
East Asian Observatory, 660 N. A'ohoku Place, Hilo, HI 96720, USA

0000-0002-8131-6730]Yosuke Mizuno
Tsung-Dao Lee Institute and School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, 200240, China
Institut für Theoretische Physik, Goethe-Universität Frankfurt, Max-von-Laue-Straße 1, D-60438 Frankfurt am Main, Germany

0000-0002-3882-4414]James M. Moran
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

0000-0003-1364-3761]Kotaro Moriyama
Massachusetts Institute of Technology Haystack Observatory, 99 Millstone Road, Westford, MA 01886, USA
Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Mizusawa, Oshu, Iwate 023-0861, Japan

0000-0002-2739-2994]Cornelia Müller
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands

0000-0003-1984-189X]Gibwa Musoke
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands

0000-0003-0329-6874]Alejandro Mus Mejías
Departament d'Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, E-46100 Burjassot, València, Spain
Observatori Astronòmic, Universitat de València, C. Catedrático José Beltrán 2, E-46980 Paterna, València, Spain

0000-0001-6920-662X]Neil M. Nagar
Astronomy Department, Universidad de Concepción, Casilla 160-C, Concepción, Chile

0000-0001-6081-2420]Masanori Nakamura
National Institute of Technology, Hachinohe College, 16-1 Uwanotai, Tamonoki, Hachinohe City, Aomori 039-1192, Japan
Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.

0000-0002-1919-2730]Ramesh Narayan
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

Gopal Narayanan
Department of Astronomy, University of Massachusetts, 01003, Amherst, MA, USA

0000-0001-8242-4373]Iniyan Natarajan
Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, Makhanda 6140, South Africa
Wits Centre for Astrophysics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg 2050, South Africa
South African Radio Astronomy Observatory, Observatory 7925, Cape Town, South Africa

0000-0002-8247-786X]Joey Neilsen
Villanova University, Mendel Science Center Rm. 263B, 800 E Lancaster Ave, Villanova PA 19085

Roberto Neri
Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint Martin d'Hères, France

0000-0003-1361-5699]Chunchong Ni
Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada
Waterloo Centre for Astrophysics, University of Waterloo, Waterloo, ON N2L 3G1 Canada

0000-0002-4151-3860]Aristeidis Noutsos
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0001-6923-1315]Michael A. Nowak
Physics Department, Washington University CB 1105, St Louis, MO 63130, USA

Hiroki Okino
Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Mizusawa, Oshu, Iwate 023-0861, Japan
Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

0000-0001-6833-7580]Héctor Olivares
Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands

0000-0002-2863-676X]Gisela N. Ortiz-León
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

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

Feryal Özel
Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA

0000-0002-7179-3816]Daniel C. M. Palumbo
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

0000-0001-6558-9053]Jongho Park
Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.

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

0000-0003-2155-9578]Ue-Li Pen
Perimeter Institute for Theoretical Physics, 31 Caroline Street North, Waterloo, ON, N2L 2Y5, Canada
Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
Canadian Institute for Advanced Research, 180 Dundas St West, Toronto, ON M5G 1Z8, Canada

0000-0002-5278-9221]Dominic W. Pesce
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

Vincent Piétu
Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint Martin d'Hères, France

Richard Plambeck
Radio Astronomy Laboratory, University of California, Berkeley, CA 94720, USA

Aleksandar PopStefanija
Department of Astronomy, University of Massachusetts, 01003, Amherst, MA, USA

0000-0002-4584-2557]Oliver Porth
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
Institut für Theoretische Physik, Goethe-Universität Frankfurt, Max-von-Laue-Straße 1, D-60438 Frankfurt am Main, Germany

0000-0002-6579-8311]Felix M. Pötzl
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0002-0393-7734]Ben Prather
Department of Physics, University of Illinois, 1110 West Green Street, Urbana, IL 61801, USA

0000-0002-4146-0113]Jorge A. Preciado-López
Perimeter Institute for Theoretical Physics, 31 Caroline Street North, Waterloo, ON, N2L 2Y5, Canada

Dimitrios Psaltis
Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA

0000-0001-9270-8812]Hung-Yi Pu
Department of Physics, National Taiwan Normal University, No. 88, Sec.4, Tingzhou Rd., Taipei 116, Taiwan, R.O.C.
Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.
Perimeter Institute for Theoretical Physics, 31 Caroline Street North, Waterloo, ON, N2L 2Y5, Canada

0000-0002-9248-086X]Venkatessh Ramakrishnan
Astronomy Department, Universidad de Concepción, Casilla 160-C, Concepción, Chile

0000-0002-1407-7944]Ramprasad Rao
Institute of Astronomy and Astrophysics, Academia Sinica, 645 N. A'ohoku Place, Hilo, HI 96720, USA

Mark G. Rawlings
East Asian Observatory, 660 N. A'ohoku Place, Hilo, HI 96720, USA

0000-0002-5779-4767]Alexander W. Raymond
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

0000-0002-1330-7103]Luciano Rezzolla
Institut für Theoretische Physik, Max-von-Laue-Strasse 1, 60438 Frankfurt, Germany
Frankfurt Institute for Advanced Studies, Ruth-Moufang-Strasse 1, 60438 Frankfurt, Germany
School of Mathematics, Trinity College, Dublin 2, Ireland

0000-0002-7301-3908]Bart Ripperda
Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, NJ 08544, USA
Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA

0000-0001-5461-3687]Freek Roelofs
Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands

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

0000-0002-2016-8746]Mel Rose
Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA

Arash Roshanineshat
Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA

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

0000-0002-1931-0135]Alan L. Roy
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0001-7278-9707]Chet Ruszczyk
Massachusetts Institute of Technology Haystack Observatory, 99 Millstone Road, Westford, MA 01886, USA

0000-0003-4146-9043]Kazi L. J. Rygl
Italian ALMA Regional Centre, INAF-Istituto di Radioastronomia, Via P. Gobetti 101, I-40129 Bologna, Italy

Salvador Sánchez
Instituto de Radioastronomía Milimétrica, IRAM, Avenida Divina Pastora 7, Local 20, E-18012, Granada, Spain

0000-0002-7344-9920]David Sánchez-Arguelles
Instituto Nacional de Astrofísica, Óptica y Electrónica. Apartado Postal 51 y 216, 72000. Puebla Pue., México
Consejo Nacional de Ciencia y Tecnología, Av. Insurgentes Sur 1582, 03940, Ciudad de México, México

0000-0001-5946-9960]Mahito Sasada
Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Mizusawa, Oshu, Iwate 023-0861, Japan
Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan

0000-0001-6214-1085]Tuomas Savolainen
Aalto University Department of Electronics and Nanoengineering, PL 15500, FI-00076 Aalto, Finland
Aalto University Metsähovi Radio Observatory, Metsähovintie 114, FI-02540 Kylmälä, Finland
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

F. Peter Schloerb
Department of Astronomy, University of Massachusetts, 01003, Amherst, MA, USA

Karl-Friedrich Schuster
Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint Martin d'Hères, France

0000-0002-1334-8853]Lijing Shao
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People's Republic of China

0000-0003-3540-8746]Zhiqiang Shen (沈志强)
Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, People's Republic of China
Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Nanjing 210008, People's Republic of China

0000-0003-3723-5404]Des Small
Joint Institute for VLBI ERIC (JIVE), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands

0000-0002-4148-8378]Bong Won Sohn
Korea Astronomy and Space Science Institute, Daedeok-daero 776, Yuseong-gu, Daejeon 34055, Republic of Korea
University of Science and Technology, Gajeong-ro 217, Yuseong-gu, Daejeon 34113, Republic of Korea
Department of Astronomy, Yonsei University, Yonsei-ro 50, Seodaemun-gu, 03722 Seoul, Republic of Korea

0000-0003-1938-0720]Jason SooHoo
Massachusetts Institute of Technology Haystack Observatory, 99 Millstone Road, Westford, MA 01886, USA

0000-0003-1526-6787]He Sun (孙赫)
California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA

0000-0003-0236-0600]Fumie Tazaki
Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Mizusawa, Oshu, Iwate 023-0861, Japan

0000-0003-3906-4354]Alexandra J. Tetarenko
East Asian Observatory, 660 N. A’ohoku Place, Hilo, HI 96720, USA

0000-0003-3826-5648]Paul Tiede
Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada
Waterloo Centre for Astrophysics, University of Waterloo, Waterloo, ON N2L 3G1 Canada

0000-0002-6514-553X]Remo P. J. Tilanus
Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands
Leiden Observatory—Allegro, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands
Netherlands Organisation for Scientific Research (NWO), Postbus 93138, 2509 AC Den Haag, The Netherlands
Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA

0000-0002-3423-4505]Michael Titus
Massachusetts Institute of Technology Haystack Observatory, 99 Millstone Road, Westford, MA 01886, USA

0000-0002-7114-6010]Kenji Toma
Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980-8578, Japan
Astronomical Institute, Tohoku University, Sendai 980-8578, Japan

0000-0001-8700-6058]Pablo Torne
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
Instituto de Radioastronomía Milimétrica, IRAM, Avenida Divina Pastora 7, Local 20, E-18012, Granada, Spain

Tyler Trent
Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA

0000-0002-1209-6500]Efthalia Traianou
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0003-0465-1559]Sascha Trippe
Department of Physics and Astronomy, Seoul National University, Gwanak-gu, Seoul 08826, Republic of Korea

0000-0001-5473-2950]Ilse van Bemmel
Joint Institute for VLBI ERIC (JIVE), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands

0000-0002-0230-5946]Huib Jan van Langevelde
Joint Institute for VLBI ERIC (JIVE), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
Leiden Observatory, Leiden University, Postbus 2300, 9513 RA Leiden, The Netherlands

0000-0001-7772-6131]Daniel R. van Rossum
Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands

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

0000-0003-1140-2761]Derek Ward-Thompson
Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK

0000-0002-8960-2942]John Wardle
Physics Department, Brandeis University, 415 South Street, Waltham, MA 02453, USA

0000-0002-4603-5204]Jonathan Weintroub
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

0000-0003-4058-2837]Norbert Wex
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0002-7416-5209]Robert Wharton
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0002-8635-4242]Maciek Wielgus
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

0000-0001-6952-2147]George N. Wong
Department of Physics, University of Illinois, 1110 West Green Street, Urbana, IL 61801, USA

0000-0003-4773-4987]Qingwen Wu (吴庆文)
School of Physics, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, People's Republic of China

0000-0001-8694-8166]Doosoo Yoon
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands

0000-0003-0000-2682]André Young
Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands

0000-0002-3666-4920]Ken Young
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

0000-0003-3564-6437]Feng Yuan (袁峰)
Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, People's Republic of China
Key Laboratory for Research in Galaxies and Cosmology, Chinese Academy of Sciences, Shanghai 200030, People's Republic of China
School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, People's Republic of China

Ye-Fei Yuan (袁业飞)
Astronomy Department, University of Science and Technology of China, Hefei 230026, People's Republic of China

0000-0001-7470-3321]J. Anton Zensus
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0002-4417-1659]Guang-Yao Zhao
Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain

0000-0002-9774-3606]Shan-Shan Zhao
Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, People's Republic of China

Gabriele Bruni
INAF - Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere 100, I-00133 Roma, Italy

0000-0003-4274-4369]A. Gopakumar
Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Mumbai 400005, India

0000-0001-7520-4305]Antonio Hernández-Gómez
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

Ruben Herrero-Illana
European Southern Observatory (ESO), Alonso de Córdova 3107, Casilla, 19001, Vitacura, Santiago de Chile, Chile
Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain

0000-0002-5311-9078]Adam Ingram
Department of Physics, Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK

S. Komossa
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0001-9303-3263]Y. Y. Kovalev
Astro Space Center of Lebedev Physical Institute, Profsoyuznaya 84/32, 117997 Moscow, Russia
Moscow Institute of Physics and Technology, Institutsky per. 9, Dolgoprudny 141700, Russia
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

0000-0002-2315-2571]Dirk Muders
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

Manel Perucho
Departament d'Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, E-46100 Burjassot, València, Spain
Observatori Astronòmic, Universitat de València, C. Catedrático José Beltrán 2, E-46980 Paterna, València, Spain

Florian Rösch
Institut für Theoretische Physik und Astrophysik Universität Würzburg, Emil Fischer, Str. 31 97074, Würzburg, Germany

Mauri Valtonen
Finnish Centre for Astronomy with ESO (FINCA), Quantum, Vesilinnantie 5, FI-20014 Turun yliopisto, Finland

We present the results from a full polarization study carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) during the first Very Long Baseline Interferometry (VLBI) campaign, which was conducted  in Apr 2017 in the $\lambda$3 mm and $\lambda$1.3 mm bands, in concert with the Global mm-VLBI Array (GMVA) and the Event Horizon Telescope (EHT), respectively.
We determine the  polarization and  Faraday  properties  of all VLBI targets, including Sgr A*, M87, and a dozen radio-loud AGN, in the two bands at several epochs in a time window of ten days.
We detect high linear polarization fractions (2–15%) and large rotation measures (RM  $>10^{3.3}-10^{5.5}$ ),
confirming the trends of previous AGN studies at mm wavelengths.
We  find that blazars are more strongly polarized than other AGN in the sample, while exhibiting (on average) an order-of-magnitude  lower RM values, consistent with the AGN viewing angle unification scheme.
For Sgr A* we report a mean RM  of $(-4.2\pm0.3) \times10^5$  at 1.3 mm,  consistent with measurements over the past decade, and, for the first time, a RM of $(-2.1\pm0.1) \times10^5$  at 3 mm,
suggesting that about half of  the Faraday rotation at 1.3 mm may occur between the 3 mm photosphere and the 1.3 mm source.
We also report the first  unambiguous measurement of RM toward the M87 nucleus at mm wavelengths, which
undergoes significant  changes in magnitude and sign reversals on a one year time-scale,
spanning the range from –1.2 to 0.3 $\times\,10^5$  at 3 mm and –4.1 to 1.5 $\times\,10^5$  at 1.3 mm.
Given this time variability, we argue that, unlike the case of Sgr A*,  the RM in M87 does not  provide an accurate estimate of the mass accretion rate onto the black hole.
We put forward a two-component model, comprised of a variable compact region  and a static extended region,
that can  simultaneously explain  the polarimetric properties observed by both the EHT  (on horizon scales) and ALMA (which observes the combined emission from both  components).
These measurements provide critical constraints for the calibration, analysis, and interpretation of simultaneously obtained VLBI data with the EHT and GMVA.

§ INTRODUCTION

Active galactic nuclei (AGN) are known to host supermassive black holes (SMBHs), which accrete gas through a disk and drive powerful relativistic jets that are observed on scales of parsecs to megaparsecs <cit.>.
Magnetic fields are believed to play a major role in the formation of such relativistic jets,
by either extracting energy from a spinning SMBH via the Blandford–Znajek mechanism <cit.>
or by tapping into the rotational energy of a magnetized accretion flow via the Blandford–Payne mechanism <cit.>.

Polarization observations are  a powerful tool to probe magnetic fields and to understand their role in black-hole mass-accretion and launching and acceleration of  relativistic AGN jets.
In fact, the radio emission from AGN and their associated jets is thought to be produced by synchrotron processes, and thus they display high intrinsic linear polarization <cit.>.
LP fractions and polarization vector orientations  can provide details on the magnetic field strength and topology.
Besides LP,  circular polarization (CP) may also be present   as a consequence of Faraday conversion of the linearly-polarized synchrotron emission    <cit.>, and can also help constraining the magnetic field configuration <cit.>.

As the linearly polarized radiation travels through  magnetized plasma, it experiences Faraday  rotation of the LP vectors. The externally magnetized plasma is also known as the “Faraday screen” and the amount of Faraday rotation is known as the “rotation measure" (RM).
If the background source of polarized emission is entirely behind (and not intermixed with) the Faraday screen, the RM can be written as an integral of the product of the electron number density ($n_e$) and the magnetic field component along the line-of-sight ($B_{||}$) via:

\begin{equation}\label{eq:RM}
\mathrm{RM} = 8.1 \times 10^5 \int n_e [\mathrm{cm^{-3}}]~ \mathbf{B} [\mathrm{G}] \cdot \mathbf{\mathrm{d}l}[\mathrm{pc}]  ~~\mathrm{rad\,m}^{-2} \,.
\end{equation}

Thus, by measuring the RM one can also constrain the electron density, $n_e$, and the magnetic field, $B_{||}$,
in the plasma surrounding SMBHs.
Under the assumption that the polarized emission is produced close to the SMBH and then Faraday-rotated in the surrounding accretion flow, the RM has been used in some cases to infer the accretion rate onto SMBHs <cit.>.
Alternatively, the polarized emission may be Faraday-rotated along the jet boundary layers <cit.>.
Therefore, Faraday rotation measurements can provide crucial constraints on magnetized accretion models and jet formation models.

RM studies are typically conducted at cm wavelengths
using the Very  Large Array (VLA) or the Very Long Baseline Array <cit.>.
However, cm wavelengths are strongly affected by synchrotron self-absorption (SSA) close to the central engines and can therefore only probe magnetized plasma in the optically thin regions at relatively larger distances (parsec scales) from the SMBH <cit.>.
On the other hand, emission at mm wavelengths is optically thin from the innermost regions of the jet base (and accretion disc), enabling us to study the plasma and magnetic fields much closer to the SMBH.
In addition, LP can be more easily detected at mm wavelengths because the mm emission region is smaller  <cit.>, and so depolarization induced by RM variations across the source (e.g., owing to a tangled magnetic field) is less significant.
Finally, since Faraday rotation is smaller at shorter wavelengths (with a typical $\lambda^2$ dependence), mm-wavelength measurements more clearly reflect the intrinsic LP properties, and therefore the magnetic field of the system.

Unfortunately, polarimetric measurements at mm wavelengths have so far been limited by sensitivity and instrumental systematics.
The first interferometric measurements of RM at (sub)mm wavelengths were conducted towards Sgr A* with the Berkeley-Illinois-Maryland Association (BIMA) array    <cit.> and the Submillimeter Array  <cit.>, which yielded a RM $\sim -5 \times10^5$ .
SMA measurements towards M87 provided an upper limit  $\left|{\rm RM}\right| < 7.5 \times10^5$  <cit.>.
Other AGN with RM detections with mm interferometers include 3C 84 with RM $=8 \times10^5$  (;
see also  for a similarly high RM measured with the VLBA at 43 GHz),
PKS 1830-211 (at a redshift $z=2.5$) with RM $ \sim 10^7$    <cit.>,
and 3C 273 with RM $=5 \times10^5$  <cit.>.
Additional examples of AGN RM studies with mm single-dish telescopes can be found in  <cit.> and  <cit.>.

In order to  progress in this field,  polarization interferometric studies at mm wavelengths should be extended to a larger sample of AGN and it will be important to investigate both time and frequency dependence effects, by carrying out observations at multiple frequency bands and epochs.
Ultimately, observational studies should be conducted at the highest possible angular resolutions in order to resolve the innermost regions of the accretion flow and/or the  base of  relativistic jets.

The advent of the Atacama Large Millimeter/ submillimeter Array (ALMA) as a phased array   <cit.> as a new element to Very Long Baseline Interferometry (VLBI)  at millimeter (mm) wavelengths (hereafter mm-VLBI) has been a game charger in terms of sensitivity and polarimetric studies.

In this paper, we present a complete polarimetric analysis of ALMA observations carried out  during the first VLBI campaign.

§.§ mm-VLBI with ALMA

The first science observations with phased-ALMA were conducted in April 2017  <cit.>, in concert with two different VLBI networks: the  Global mm-VLBI Array (GMVA) operating at 3 mm wavelength <cit.> and the Event Horizon Telescope (EHT)  operating at 1.3 mm wavelength  <cit.>.
These observations had two “key science” targets,
the SMBH candidate at the Galactic center, Sgr A*, and  the nucleus of the giant elliptical galaxy M87 in the Virgo cluster, M87*, both enabling studies at horizon-scale resolution <cit.>.
In addition to those targets, VLBI observations with phased-ALMA also targeted a sample of  a dozen radio-loud AGN, including the closest and most luminous quasar 3C 273, the bright $\gamma$-ray-emitting  blazar 3C 279, the closest radio-loud galaxy Centaurus A (Cen A), and the best supermassive binary black hole candidate OJ287.

In 2019, the first EHT observations with phased-ALMA yielded groundbreaking results, most notably
the first ever event-horizon-scale image of the M87* SMBH <cit.>.

Beyond this breakthrough, EHT observations have now  imaged polarized emission in the ring surrounding M87*, resolving for the first time the magnetic field structures within a few Schwarzschild radii ()  of a SMBH <cit.>. In addition, these new polarization images enable us to place tight constraints on physical models of the magnetized accretion flow around the M87* SMBH and, in general, on relativistic jet launching theories <cit.>.

Both the VLBI imaging and the theoretical modelling use constraints  from ALMA observations <cit.>. In fact, besides providing a huge boost in sensitivity and uv-coverage <cit.>, the inclusion of ALMA in a VLBI array provides another important advantage:
standard interferometric visibilities among the ALMA antennas are computed by the ALMA
correlator and simultaneously stored in the  ALMA archive together with the VLBI recording of the phased signal   <cit.>.
Furthermore, VLBI observations are always performed in full-polarization mode in order to supply the inputs to the polarization conversion process (from linear to circular) at the VLBI correlators,
which is carried out using the PolConvert software <cit.> after
the "Level 2 Quality Assurance"  (QA2) process <cit.>.
Therefore, VLBI observations with ALMA yield  a full-polarization interferometric dataset,
which provides both source-integrated information for  refinement and validation of VLBI data calibration <cit.> as well as observational constraints to theoretical models  <cit.>.
Besides these  applications, this dataset carries valuable scientific value on its own and can be used to derive  mm emission,  polarization, and Faraday properties of  a selected sample of  AGN  on arcsecond scales.

§.§ This paper

In this paper, we present a full polarization study carried out with ALMA in the $\lambda$3 mm and $\lambda$1 mm bands towards Sgr A*, M87, and  a dozen radio-loud AGN, with particular emphasis on their polarization and Faraday properties.
The current paper is structured as follows.

Section <ref> summarizes the 2017 VLBI observations (<ref>), the procedures followed for the data calibration (<ref>),  the details of the full-polarization image deconvolution (<ref>), and additional observations on M87 (<ref>).

Section <ref> describes the procedures of data analysis. After presenting some representative total-intensity images of Sgr A* and M87   (<ref>),  two independent algorithms to estimate the Stokes parameters of the compact cores are described (<ref>).
The Stokes parameters for each source and spectral window are then converted into fractional LP and EVPA (<ref>),
and used to estimate Faraday rotation (<ref>) and (de)polarization  effects (<ref>). Finally, the CP analysis is summarised in <ref>.

Section <ref>  reports the polarimetric and Faraday properties of all the GMVA and EHT target sources, with dedicated subsections on AGN, M87, and Sgr A*.

In Section <ref>, the polarization properties presented in the previous sections are used to explore potential physical origins of the polarized emission and location of Faraday screens in the context of  SMBH accretion and  jet formation models. <ref> presents a comparison between the $\lambda$3 mm and $\lambda$1.3 mm bands, including a discussion on the effects of synchrotron opacity and Faraday rotation; <ref> presents a comparison between the case of blazars and other AGN; <ref> discusses about depolarization in radio galaxies and its possible connection to instrumental effects.
<ref> is devoted to the special case of M87, including a discussion about the origin of the Faraday screen (internal vs. external; <ref>) as well as a simple two-component Faraday model (<ref>).
Finally, <ref> is dedicated to the special case of Sgr A*.

Conclusions are drawn in Section <ref>.

The paper is supplemented with a number of appendices including:  the list of ALMA projects observed during the VLBI campaign in April 2017 (Appendix <ref>), a full suite of polarimetric images  (<ref>)    for all the observed targets,
comparisons between multiple flux-extraction methods (<ref>) and between the polarimetry results obtained during the VLBI campaign and the monitoring programme with the Atacama Compact Array (<ref>),   tables with polarimetric quantities  per ALMA spectral-window (<ref>),
Faraday RM plots  (<ref>),
quality assessment of the circular polarization estimates (<ref>), and
mm spectral indices  of all the observed targets (<ref>).
Finally, a two-component polarization model for M87, which combines constraints from ALMA and EHT observations, is presented in Appendix <ref>.

§ OBSERVATIONS, DATA PROCESSING, AND IMAGING

§.§ 2017 VLBI observations with ALMA

The observations with phased-ALMA were  conducted as part of Cycle 4 during the 2017 VLBI campaign
in ALMA Band 3 (April 1-3) and Band 6 (April 5-11), respectively.
The ALMA data were acquired simultaneously with the VLBI observations (in this sense they are a “byproduct" of the VLBI operations).
The ALMA array was in the compact configurations C40-1 (with 0.15 km longest baseline)  and, after Apr 6, C40-3 (with 0.46 km longest baseline).
Only antennas within a  radius of 180 m (from the array center) were used for phasing on all days.
About 37 antennas were normally phased together, which is equivalent to a telescope of 73 m diameter[A few more antennas participated in the observations without being phased, the so-called “comparison” antennas, which are mostly used to provide feedback on the efficiency of the phasing process <cit.>.].
In both Band 3 and 6,
the spectral setup includes four spectral windows (SPWs) of 1875 MHz,
two in the lower and two in the upper sideband,
correlated with 240 channels per SPW (corresponding to a spectral resolution of 7.8125 MHz[The recommended continuum setup for standard ALMA observations in full polarization mode is somewhat different and consists of 64 channels,  31.25 MHz wide, per SPW.]).
In Band 3 the four SPWs are centered at 86.268, 88.268, 98.328, and 100.268 GHz[

The "uneven" frequency separation with SPW=2 is due to constraints on the first and second Local Oscillators  in the ALMA's tuning system.]
while in Band 6 they are centered at 213.100, 215.100, 227.100, and 229.100 GHz.

Three projects were observed in Band 3 with the GMVA
(science targets: OJ 287, Sgr A*, 3C 273)
and six projects were observed in Band 6 with the EHT (science targets: OJ 287, M87, 3C 279, Sgr A*, NGC 1052, Cen A).
The projects were arranged and calibrated in “tracks” (where one track consists of the observations taken during the same day/session).
In Appendix <ref> we provide a list of the observed projects and targets on each day,
with the underlying identifications of (calibration and science target) sources within each project
(see Tables <ref> and <ref> in Appendix <ref>).
More details of the observation structure and calibration sources can be found in <cit.>.

§.§ Data calibration and processing

During phased-array operations, the data path from the antennas to the ALMA correlator is different with respect to standard interferometric operations <cit.>.
This makes the  calibration of VLBI observations within the Common Astronomy Software Applications (casa) package intrinsically different and some essential modification in the procedures is required with respect to ALMA standard  observations.
The special steps added to the standard ALMA  polarization calibration procedures <cit.> are described in detail in <cit.>.
The latter focus mostly on the LP calibration and the polarization conversion at the VLBI correlators <cit.>. In this paper we extend the data analysis also to the CP.

Only sources observed in VLBI mode were
calibrated in polarization <cit.>.
Therefore the sources exclusively observed for ordinary ALMA calibration during the VLBI schedule gaps  (i.e.,  and  calibrators) are excluded from this analysis (compare the source list in
Tables <ref> and <ref> in Appendix <ref>
with Tables 4 and 6 in ).
Two additional sources observed on Apr 7, 3C 84 and J0006-0623, are also excluded from the following analysis. These sources are in fact flagged in a final flagging step (run on the fully calibrated $uv$-data before imaging and data analysis), which
removes visibility data points having amplitudes outside  a certain range (set  by  three times the RMS from the median of the  data) and
a source elevation below 25.
Finally,  the two weakest targets observed at 1.3 mm, NCG 1052 and J0132-1654,
were found to fall below the flux threshold
(correlated flux density of $>$0.5 Jy on intra-ALMA baselines)
required to enable on-source phasing of the array as commissioned  <cit.>.
Despite   these two sources are detected with high signal-to-noise ratio (SNR) in total intensity (SNR$>1000$) and polarized flux (SNR$>50$ for J0132-1654), we recommend extra care in interpreting these source measurements owing to lower data quality.

§.§ Full-stokes imaging

All targets observed in Band 3 and Band 6 are imaged using the casa task  in all Stokes parameters: $I$, $Q$, $U$, $V$.
A Briggs weighting scheme <cit.> is adopted with a robust parameter of 0.5, and a cleaning gain of 0.1.
A first quick cleaning (100 iterations over all four Stokes parameters) is done in the inner 10 and 4  in bands 3 and 6, respectively.
Providing there is still significant emission ($>7\sigma$) in the residual maps (e.g., in M87 and Sgr A*), an automatic script changes the cleaning mask  accordingly, and a second, deeper cleaning is done down to 2$\sigma$ (these two clean steps are run with parameter ).
A final interactive clean step (with ) is run to adjust the mask to include real emission which was missed by the automatic masking and to clean deeper sources with complex structure and high-signal residuals (this step was essential for proper cleaning of Sgr A*).
No self-calibration was attempted during the imaging stage (the default calibration scheme for ALMA-VLBI data already relies on self-calibration; see  for details).

We produced maps of size 256$\times$256 pixels, with a pixel size of 05 and 02 in Band 3 and Band 6, respectively, resulting in maps with a field-of-view (FOV) of 128$\times$ 128 and 51$\times$ 51, respectively, thereby comfortably covering the primary beams of ALMA Band 3 (60) and Band 6 (27) antennas.
We produced maps for individual SPWs and by combining SPWs in each sideband (SPW=0,1 and SPW=2,3), setting the  parameters  and , as well as by combining all four SPWs, using   and .
The latter achieved better sensitivity and yielded higher quality images[The  performed best when combining all four SPW, yielding on average 30–40% better sensitivity than  combining two SPW at a time, as expected for RMS $\propto 1/\sqrt{\Delta \nu}$. However,  performed poorly when combining all four SPW, especially for steep spectral index sources, yielding up to 50% worse RMS than .], so we used the combined SPW images for the imaging analysis presented in this paper (except for the per-SPW analysis).

Representative total-intensity images in Band 3  and  Band 6 are shown in Figures <ref> (Stokes $I$) and <ref> (Stokes $I$ + polarized intensity),  whereas the full suite of images including each source observed in Band 3  and  Band 6 on each day of the 2017 VLBI campaign is reported in Appendix <ref> (Figures <ref>–<ref>).

The array  configurations employed during phased-array observations yielded synthesized beams in the range
[47–61] $\times$ [24–34]  in Band 3
[12–30] $\times$ [07–15]
in Band 6 (depending on the day and the target).
Images on different days achieve different sensitivities and angular resolutions, depending on the time on-source and baseline lengths of the phased-array.
In particular, the relatively large range of beamsizes in Band 6  is due to the fact that,
during the EHT campaign, progressively more antennas were moved out from the “central cluster” (with a diameter $<$ 150 m).
As a consequence,  in the last day of the campaign (Apr 11) the observations were carried out with a more extended array, yielding a beamsize in the range [12–15] $\times$ [07–09]
(i.e., an angular resolution  roughly two times better than that of other tracks).
Tables <ref> and <ref> in Appendix <ref>     report the synthesized beamsize and the RMS achieved in the images of each Stokes parameter for each source observed in Band 3  and  Band 6 on each day.

§.§ Additional ALMA polarization datasets on M87

In addition to the April 2017 data,  we have also analysed  ALMA data acquired during the 2018 VLBI campaign as well as ALMA archival polarimetric experiments targeting M87.

The  2018 VLBI campaign was conducted as part of Cycle 5 in Band 3 (April 12-17) and Band 6 (April 18-29), respectively.
The observational setup  was the same as in Cycle 4, as outlined in <ref> (a full description of the 2018 VLBI campaign will be reported elsewhere).
Three observations of M87 at $\lambda$1.3 mm were conducted on Apr 21, 22, and 25 under the  project 2017.1.00841.V.
For the data processing and calibration, we followed the same procedure used for the 2017 observations, as outlined in <ref>.

The archival experiments include three observations at $\lambda$3mm carried out on Sep and Nov 2015 (project codes: 2013.1.01022.S and 2015.1.01170.S, respectively) and Oct 2016 (project code: 2016.1.00415.S), and one observation at $\lambda$1.3mm from Sep 2018 (project code: 2017.1.00608.S).

For projects 2013.1.01022.S and  2015.1.01170.S,  we used directly the imaging products released with the standard QA2 process and publicly available for download from the ALMA archive.
For  projects 2016.1.00415.S and  2017.1.00608.S,
we downloaded the raw visibility data and the QA2 calibration products from the ALMA archive, and we revised the polarization calibration after additional data flagging, following the procedures outlined in <cit.>.

The data imaging was performed following the same procedures outlined in <ref>.
After imaging,  we found that in 2017.1.00608.S,
Stokes $I$, $Q$, and $U$ are not co-located: $U$ is shifted $\sim$0.07 to the East, while $Q$ is shifted $\sim$0.13 West and $\sim$0.07 north, with respect to $I$, respectively.
This shift (whose origin is unknown) prevents us to assess reliably the polarimetric properties of M87. Therefore, we will not use 2017.1.00608.S in the analysis presented in this paper.
The analysis and results of the other datasets will be presented in <ref>.

§ DATA ANALYSIS

Representative total intensity images.
Left panel:
Image of Sgr A* at  3 mm on Apr 3 (grey-scale and blue contour) and at  1.3 mm (yellow contours) on Apr 6 2017.
The image showcases the well-known “mini-spiral" structure surrounding the central compact core, including  the eastern and northern arms, the western arc, and the bar at the center.
The contour levels at 1.3 mm are $5\sigma \times 2^n$ where $\sigma=$0.44 mJy beam$^{-1}$
and n$=0\,,1\,,2\,,3\,\ldots$ up to the peak flux-density; the contour level at 3 mm corresponds to  $20\sigma$ ($\sigma=$0.8 mJy beam$^{-1}$).
The peak flux-density is $2.5\,\pm\,0.1$ ($2.6\,\pm\,0.3$) Jy beam$^{-1}$ and the integrated flux density across the entire source is $9.9\,\pm\,0.5$ ($4.9\,\pm\,0.5$) Jy at a representative frequency of 93 (221) GHz.
The FOV is given by the primary beam in Band 3 ($\sim$60) and 1 pc corresponds to 24.
The beamsizes are 50 $\times$ 27 (P.A.= –81.1) in Band 3 and 22 $\times$ 13 (P.A.= -77.5) in Band 6, shown as a blue and yellow ovals, respectively, in the lower left corner.

Right  panel:
Image of M87  at 1.3 mm  on Apr 11 2017. The image showcases the structure of the kpc-scale  relativistic  jet comprised of a  bright core at the nucleus and the  knots along the jet labeled as D, F, A, B, C; HST-1 is not resolved from the nucleus in these images.
The RMS noise level is 0.16 mJy beam$^{-1}$, and the contour levels are a factor of 10 and 40 of the RMS.
The peak flux-density is 1.34 Jy beam$^{-1}$ and the integrated flux-density is 1.57 Jy at a representative frequency of 221 GHz.
The FOV is given by the primary beam in Band 6 ($\sim$ 27 at 1.3 mm).
1 kpc  corresponds to 12.
This observation was conducted with the most extended array during the VLBI campaign, yielding the highest angular resolution (beam size = 12 $\times$ 08,  P.A.= 79.3, shown in the lower left corner).

In both panels the four observing SPWs (see  <ref>) were used together for imaging. The intensity brightness is plotted using  a logarithmic weighting function (starting from the $5\sigma$-level), in order to highlight the full extent of both the mini-spiral (in Sgr A*)  and the jet (in M87).

§.§ Representative total intensity images

The sources targeted by the GMVA and EHT are generally unresolved  at
arcsecond scales and their images are mostly consistent with point sources (see images displayed in Appendix <ref>).
The EHT key science targets, Sgr A* and M87, are clear  exceptions, and show  complex/extended structures  across  tens of arcseconds.
We show representative images of  Sgr A* (3 mm, Apr 3; 1.3 mm, Apr 6) and M87 (1.3 mm, Apr 11)  in Figure <ref>.
The images displayed cover an area corresponding to the primary beam of the ALMA antennas (27 in Band 6 and 60 in Band 3; the correction for the attenuation of the primary beam is not  applied to these maps).

The Sgr A* images clearly depict the well-known “mini-spiral” structure which traces ionized gas streams surrounding the central compact source; the mini-spiral has been studied in a wide range of wavelengths  <cit.>.
The  “eastern arm”, the “northern arm”, and the “bar” are clearly seen in both Band 3 and Band 6, while the “western arc” is clearly traced only in the Band 3 image (it falls mostly outside of the antenna primary beam for Band 6).
Similar images were obtained in the 100, 250, and 340 GHz bands in ALMA Cycle 0 by <cit.>.
Since Sgr A* shows considerable variability in its core at  mm-wavelengths <cit.>, the displayed maps and quoted flux values throughout this paper should be considered as time-averaged images/values at the given epoch.

The M87 jet has been observed across the entire electromagnetic spectrum <cit.>, and imaged in detail at radio wavelengths from $\lambda$1 metre <cit.> through $\lambda$[15–0.7] cm  <cit.> up to $\lambda$ 3 mm  <cit.>.
VLA images at lower radio frequencies <cit.> showcase a bright component at the nucleus and a kiloparsec-scale (kpc-scale) relativistic jet, extending across  approximately 25 ($\sim 2$ kpc) from  the central core.
Images of the kpc-scale relativistic jet were also produced with ALMA Cycle-0 observations at $\lambda$ 3 mm <cit.> and with the SMA at $\lambda$ 1 mm <cit.>, but could only recover the bright central core and the strongest knots along the jet.

Our $\lambda$1.3 mm ALMA image showcases a similar structure, but the higher dynamic range (when compared with these earlier studies) allows us to recover the continuous  structure of the   straight and narrow kpc-scale  jet across approximately 25 from the nucleus, including knots D, F, A, B, C, at increasing distance from the central core (HST-1 is not resolved from the nucleus in these images).
The jet structure at larger radii ($\gtrsim 2$ kpc) as well as the jet-inflated radio lobes, imaged in great detail with observations at lower frequencies, are not recovered in our images (see for example the NRAO 20 cm VLA image).

§.§ Extracting Stokes parameters in the compact cores

We extract flux values for Stokes $I$, $Q$, $U$, and $V$ in the compact cores of each target observed in Band 3 and Band 6.
We employ three different methods which use
both the visibility data and the full-Stokes images.

In the $uv$-plane analysis, we use the external casa library uvmultifit
To reduce its processing time, we first average all (240) frequency channels to obtain  one-channel four-SPW visibility $uv$-files.
We assume that the emission is dominated by a central point source at the phase centre and  we  fit a  function to the visibilities to obtain  Stokes $I$, $Q$, $U$, and $V$ parameters in each individual SPW.
Uncertainties are assessed with Monte Carlo (MC) simulations, as the standard deviation of 1000 MC simulations for each Stokes parameter.

For the image-based values, we take the sum of the central nine pixels of the   model component map (an area of $3 \times 3$ pixels, where the pixel size is 02 in Band 6 and 05 in Band 3).
Summing only the central pixels in the model maps allows one to isolate the core emission from the surroundings in sources with extended structure.

A third independent method  provides the integrated flux by fitting a Gaussian model to the compact source at the phase-center in each image with the  casa task .
In the remaining of this paper, we will indicate these three methods as uvmf, 3x3, and intf.

From a statistical perspective, any fitting method in the visibility domain should be statistically  more reliable  than a $\chi^2$-based fitting analysis in the image-plane (whose pixels have correlated noise), and should therefore be preferred to image-based methods.
However, we have two reasons for considering both approaches in this study:
(i) some of our targets exhibit prominent emission structure at arc-second scales (see Figure <ref>, and the maps in Appendix <ref>),
(ii) the observations are carried out
with various array configurations, resulting in a different degree of filtering of the source extended emission. Both elements can  potentially bias  the flux values of  the compact cores extracted in the visibility domain vs. the image domain.

In Appendix <ref> we  present a comparative analysis of three flux-extraction methods to assess the magnitude of such systematic biases (reported in Table <ref>).
The statistical analysis shows that the Stokes $I$ values estimated  with uvmf are consistent with those estimated from the images, with a median absolute deviation (MAD) $\leq 0.07$%  and individual offsets $<$1% (for both  point sources and  extended sources) in the case of the 3x3 method (the agreement is slightly worse for the intf method).
These deviations are negligible when compared to the absolute uncertainty of ALMA's flux calibration (10% in Band 6).
This consistency generally holds also for  Stokes $Q$ and $U$ (with MAD $< 1$%) and other derived parameters within their  uncertainties (see Table <ref>).
We therefore conclude that, for the purpose of the polarimetric analysis conducted in this paper, the uv-fitting method  uvmf provides  sufficiently precise flux values  for the Stokes parameters (but see Appendix <ref>  for details on M87 and  Sgr A*).

<cit.> report the Stokes $I$ flux values per source  estimated in the uv-plane from amplitude gains using the casa task .
We assess that the Stokes $I$ estimated from the visibilities with uvmf are consistent with those estimated with  generally within 1%.
In addition, <cit.> compared the  flux values (after opacity correction) with the predicted values from the regular flux monitoring programme with the ALMA Compact Array (ACA), showing that these values are generally within 10% (see their Appendix B and their Fig. 16).
In Appendix <ref> we perform a similar comparative analysis for the sources commonly observed in the ALMA-VLBI campaign and the AMAPOLA polarimetric Grid Survey, concluding that our polarimetric measurements are generally consistent with historic trends of grid sources (see Appendix <ref> for more details and comparison plots).

Frequency-averaged polarization properties of GMVA targets (at a representative frequency of 93 GHz).

Source     Day     I     Spectral Index     LP     EVPA$^{a}$     $\chi_0$     RM     Depol.

[2017]      [Jy]     $\alpha$      [%]     [deg]     [deg]     [$10^5$ rad m$^{-2}$]     [$10^{-4}$  GHz$^{-1}$]

OJ287                  Apr 2              5.97$\pm$0.30                     -0.619$\pm$0.029                       8.811$\pm$0.030                       -70.02$\pm$0.10                  -71.85$\pm$0.37                    0.0305$\pm$0.0062         2.244$\pm$0.071

J0510+1800                  Apr 2              3.11$\pm$0.16                     -0.6360$\pm$0.0059                       4.173$\pm$0.030                       81.86$\pm$0.21                  65.49$\pm$0.81                    0.273$\pm$0.013         2.639$\pm$0.078

4C 01.28                  Apr 2              4.86$\pm$0.24                     -0.480$\pm$0.033                       4.421$\pm$0.030                       -32.27$\pm$0.19                  -31.73$\pm$0.74                    -0.009$\pm$0.012         2.117$\pm$0.054

Sgr A*                  Apr 3              2.52$\pm$0.13                     -0.08$\pm$0.13                       0.735$\pm$0.030                       8.1$\pm$1.4                  135.4$\pm$5.3                    -2.13$\pm$0.10         4.72$\pm$0.13

J1924-2914                  Apr 3              5.11$\pm$0.26                     -0.462$\pm$0.026                       4.841$\pm$0.030                       -46.38$\pm$0.18                  -46.68$\pm$0.70                    0.005$\pm$0.012         2.34$\pm$0.22

NRAO 530                  Apr 3              2.74$\pm$0.14                     -0.588$\pm$0.010                       0.921$\pm$0.030                       38.8$\pm$1.0                  51.5$\pm$3.7                    -0.213$\pm$0.061         0.4372$\pm$0.0034

4C 09.57                  Apr 3              2.85$\pm$0.14                     -0.3056$\pm$0.0057                       4.069$\pm$0.030                       -28.47$\pm$0.21                  -31.15$\pm$0.83                    0.045$\pm$0.014         0.43$\pm$0.11

3C279                  Apr 4              12.93$\pm$0.65                     -0.3703$\pm$0.0087                       12.159$\pm$0.030                       43.906$\pm$0.070                  44.98$\pm$0.27                    -0.0179$\pm$0.0045         0.456$\pm$0.041

3C273                  Apr 4              9.86$\pm$0.49                     -0.2887$\pm$0.0049                       3.984$\pm$0.030                       -45.45$\pm$0.22                  -41.87$\pm$0.85                    -0.060$\pm$0.014         -2.06$\pm$0.38

aThe EVPAs are the frequency-averaged $\bar{\chi}$ (as defined in Eq. <ref>).

Frequency-averaged polarization properties of EHT targets (at a representative frequency of 221 GHz).

Source     Day     I     Spectral Index      LP     EVPA$^{a}$     $\chi_0$     RM      Depol.

[2017]      [Jy]     $\alpha$     [%]     [deg]     [deg]     [$10^5$ rad m$^{-2}$]     [$10^{-4}$  GHz$^{-1}$]

3C279                  Apr 5              8.99$\pm$0.90                     -0.642$\pm$0.019                       13.210$\pm$0.030                       45.180$\pm$0.060                  45.20$\pm$0.51                    -0.002$\pm$0.048         0.242$\pm$0.051

3C279                  Apr 6              9.36$\pm$0.94                     -0.619$\pm$0.033                       13.010$\pm$0.030                       43.340$\pm$0.070                  43.41$\pm$0.52                    -0.007$\pm$0.049         0.303$\pm$0.018

3C279                  Apr 10              8.56$\pm$0.86                     -0.6090$\pm$0.0030                       14.690$\pm$0.030                       40.140$\pm$0.060                  40.10$\pm$0.46                    0.004$\pm$0.043         0.473$\pm$0.033

3C279                  Apr 11              8.16$\pm$0.82                     -0.683$\pm$0.019                       14.910$\pm$0.030                       40.160$\pm$0.060                  40.15$\pm$0.46                    0.001$\pm$0.043         1.027$\pm$0.015

M87                  Apr 5              1.28$\pm$0.13                     -1.212$\pm$0.038                       2.420$\pm$0.030                       -7.79$\pm$0.36                  -14.6$\pm$2.8                    0.64$\pm$0.27         1.318$\pm$0.031

M87                  Apr 6              1.31$\pm$0.13                     -1.112$\pm$0.011                       2.160$\pm$0.030                       -7.60$\pm$0.40                  -23.6$\pm$3.1                    1.51$\pm$0.30         0.888$\pm$0.046

M87                  Apr 10              1.33$\pm$0.13                     -1.171$\pm$0.023                       2.740$\pm$0.030                       0.03$\pm$0.31                  2.5$\pm$2.5                    -0.24$\pm$0.23         0.540$\pm$0.048

M87                  Apr 11              1.34$\pm$0.13                     -1.208$\pm$0.019                       2.710$\pm$0.030                       -0.64$\pm$0.32                  3.5$\pm$2.5                    -0.39$\pm$0.24         1.553$\pm$0.064

Sgr A*                  Apr 6              2.63$\pm$0.26                     -0.0270$\pm$0.0030                       6.870$\pm$0.030                       -65.83$\pm$0.13                  -14.7$\pm$1.0                    -4.84$\pm$0.10         3.75$\pm$0.10

Sgr A*                  Apr 7              2.41$\pm$0.24                     -0.057$\pm$0.059                       7.230$\pm$0.030                       -65.38$\pm$0.12                  -18.77$\pm$0.93                    -4.412$\pm$0.088         3.33$\pm$0.12

Sgr A*                  Apr 11              2.38$\pm$0.24                     -0.1450$\pm$0.0080                       7.470$\pm$0.030                       -49.33$\pm$0.12                  -14.66$\pm$0.92                    -3.281$\pm$0.087         2.52$\pm$0.32

J1924-2914                  Apr 6              3.25$\pm$0.32                     -0.780$\pm$0.012                       6.090$\pm$0.030                       -49.28$\pm$0.14                  -53.6$\pm$1.1                    0.41$\pm$0.10         0.13$\pm$0.20

J1924-2914                  Apr 7              3.15$\pm$0.31                     -0.8510$\pm$0.0070                       5.970$\pm$0.030                       -49.22$\pm$0.15                  -52.1$\pm$1.2                    0.27$\pm$0.11         0.1470$\pm$0.0080

J1924-2914                  Apr 11              3.22$\pm$0.32                     -0.677$\pm$0.031                       4.870$\pm$0.030                       -51.82$\pm$0.18                  -56.2$\pm$1.4                    0.42$\pm$0.13         0.16$\pm$0.21

OJ287                  Apr 5              4.34$\pm$0.43                     -0.91$\pm$0.10                       9.020$\pm$0.030                       -61.190$\pm$0.090                  -62.32$\pm$0.73                    0.108$\pm$0.069         0.11$\pm$0.63

OJ287                  Apr 10              4.22$\pm$0.42                     -0.781$\pm$0.088                       7.000$\pm$0.030                       -61.81$\pm$0.12                  -62.6$\pm$1.0                    0.077$\pm$0.091         0.09$\pm$0.61

OJ287                  Apr 11              4.26$\pm$0.43                     -0.715$\pm$0.043                       7.150$\pm$0.030                       -59.61$\pm$0.12                  -62.97$\pm$0.92                    0.317$\pm$0.087         0.110$\pm$0.049

4C 01.28                  Apr 5              3.51$\pm$0.35                     -0.73$\pm$0.16                       5.900$\pm$0.030                       -23.18$\pm$0.15                  -22.5$\pm$1.1                    -0.06$\pm$0.11         0.58$\pm$0.20

4C 01.28                  Apr 10              3.59$\pm$0.36                     -0.679$\pm$0.079                       5.080$\pm$0.030                       -16.82$\pm$0.17                  -16.3$\pm$1.3                    -0.05$\pm$0.12         0.68$\pm$0.26

4C 01.28                  Apr 11              3.57$\pm$0.36                     -0.630$\pm$0.024                       5.000$\pm$0.030                       -14.74$\pm$0.18                  -18.2$\pm$1.4                    0.33$\pm$0.13         0.416$\pm$0.054

NRAO 530                  Apr 6              1.61$\pm$0.16                     -0.96$\pm$0.14                       2.350$\pm$0.030                       51.59$\pm$0.37                  51.7$\pm$2.9                    -0.01$\pm$0.28         0.940$\pm$0.062

NRAO 530                  Apr 7              1.57$\pm$0.16                     -0.812$\pm$0.017                       2.430$\pm$0.030                       50.67$\pm$0.36                  51.1$\pm$2.8                    -0.04$\pm$0.27         0.82$\pm$0.15

J0132-1654                  Apr 6              0.420$\pm$0.040                     -0.625$\pm$0.086                       1.990$\pm$0.050                       15.54$\pm$0.67                  23.4$\pm$5.3                    -0.74$\pm$0.50         0.04$\pm$0.40

J0132-1654                  Apr 7              0.410$\pm$0.040                     -0.75$\pm$0.10                       2.010$\pm$0.050                       17.85$\pm$0.78                  14.3$\pm$6.2                    0.34$\pm$0.58         -0.18$\pm$0.21

NGC 1052                  Apr 6              0.430$\pm$0.040                     -0.83$\pm$0.11                       0.120$\pm$0.030                       - - - -                 - - - -                                  - - - -                   - - - -

NGC 1052                  Apr 7              0.380$\pm$0.040                     -1.33$\pm$0.16                       0.160$\pm$0.040                       - - - -                 - - - -                                  - - - -                   - - - -

Cen A                  Apr 10              5.66$\pm$0.57                     -0.197$\pm$0.038                       0.070$\pm$0.030                       - - - -                 - - - -                                  - - - -                   - - - -

3C273                  Apr 6              7.56$\pm$0.76                     -0.705$\pm$0.024                       2.390$\pm$0.030                       -55.50$\pm$0.36                  -82.2$\pm$2.8                    2.52$\pm$0.27         -2.54$\pm$0.11

J0006-0623                  Apr 6              1.99$\pm$0.20                     -0.789$\pm$0.059                       12.530$\pm$0.030                       16.480$\pm$0.070                  15.83$\pm$0.57                    0.061$\pm$0.054         0.78$\pm$0.27

aThe EVPAs are the frequency-averaged $\bar{\chi}$ (as defined in Eq. <ref>).

Polarization images of Sgr A*  (left panel) and M87 (right panel) at 1.3 mm  on Apr 6 2017.
The raster image and blue contour   show the total intensity emission, the orange contours show the linearly polarized emission, and the black vectors showcase the orientation of the EVPAs  (their length is linearly proportional to the polarized flux).
The total intensity brightness is plotted using  a logarithmic weighting function (starting from the $1\sigma$-level), the blue contour corresponds to  $5\sigma$ (where $\sigma$ is the Stokes I map RMS),
while the orange contour levels are $5\sigma \times 2^n$ (where $\sigma$ is the LP map RMS and n=0,1,2,3... up to the peak in the image). The LP fraction  at the peak of the compact core is reported in the  upper left corner in each panel.
The EVPAs are plotted every 8 pixels (16 or about 1 per beam) for Sgr A* and every 4 pixels (08 or about 2 per beam) for M87 (in order to sample more uniformly the jet).

According to the measured RM, the EVPAs towards the compact core should be rotated by $-23$ (east of north) in Sgr A* and by $-16$   in M87.
The beamsizes (shown as an oval in the lower left corner) are  22 $\times$ 13 (P.A. –77) and  22$\times$15 (P.A. –69) in the left and right panels, respectively.
Note that there are several tiny EVPAs plotted across the mini-spiral, apparently locating regions with polarized flux above the  image RMS noise cutoff ($5\sigma$). The LP and EVPA errors are however dominated by the systematic leakage (0.03% of I onto QU), which is not added to the images.  Once these systematic errors are added, the LP flux in those points falls below the $3\sigma$ measurement threshold. Therefore we do not claim detection of polarized emission outside of the central core in Sgr A*.
Besides, only the polarisation within the inner 1/3 of the primary beam is guaranteed by ALMA.
The full set of 1.3 mm observations of Sgr A*  and M87  are reported in Figures <ref> and <ref>, respectively.

Polarization properties of selected EHT targets observed during the 2017 VLBI campaign as a function of observing day.
For each source (labeled at the top of each column), the top  panel shows Stokes I (in Jy), the second panel shows the LP degree (in %), the third  panel shows the EVPA (in degrees), the fourth  panel shows the RM (in units of 10$^4$ rad m$^{-2}$), and the bottom panel shows the depolarization (in units of 10$^{-4}$ GHz$^{-1}$).
The different symbols and colors in the upper three panels indicate 4 different observing bands centered at 213 GHz  (black), 215 GHz  (blue), 227 GHz  (green), and 229 GHz (red), corresponding to SPW=0,1,2,3. 3C 279 was used as polarization calibrator in all days, except on Apr 7  when J1924-2914 was used.

§.§ Polarimetric data analysis

In this section we use the measured values of  the Stokes parameters
to determine the  polarization properties for all targets, including the fractional LP ( <ref>), the electric vector position angle (EVPA) and its variation as a function of frequency or Faraday Rotation ( <ref>), the degree of depolarization ( <ref>),
and the fractional CP ( <ref>).

These polarization quantities, averaged across the four SPWs, are reported in
Tables <ref> and <ref> for each target observed with the GMVA and the EHT, respectively (while Table <ref> summarizes all the ALMA polarimetric observations towards M87 analysed in this paper).
For selected EHT targets, the polarization properties (per SPW and per day)  are displayed in Figure <ref>.

§.§.§ Linear polarization and EVPA

The values estimated for Stokes Q and U can be combined to directly provide the fractional LP  in the form  $\sqrt{Q^{2}+U^{2}}/I$, as well as the EVPA, $\chi$, via the equation  $2\chi=\mathrm{arctan}(U/Q)$.
Tables <ref> and  <ref> in Appendix <ref> report Stokes parameters, LP, and EVPA, for each SPW. The  LP has been debiased in order to correct for the LP bias in the low SNR regime (this correction is especially relevant for low polarization sources; see Appendix <ref> for the debiased LP derivation).

The estimated LP fractions  range from $\lesssim 0.1-0.2$% for the most weakly polarized targets (Cen A and NGC 1052) to 15% for the most strongly polarized target (3C279), consistent with previous measurements (see Appendix <ref>).
The uncertainties in LP include the fitting (thermal) error of Stokes $Q$ and $U$ and
the (systematic) Stokes $I$ leakage onto Stokes $Q$ and $U$ (0.03% of Stokes $I$) added in quadrature.
This  analysis yields LP uncertainties $< 0.1\%$, similar to those quoted in previous studies <cit.>.

Figure <ref> showcases representative polarization images of Sgr A*  (left panel) and M87 (right panel) as observed at 1.3 mm  on Apr 6. The  individual images display the measured EVPAs overlaid on  the polarized flux contour images and the total intensity images.
Note that the EVPAs are not Faraday-corrected and that the  measured[The actual magnetic field in the source may be different from the measured one, which can be affected by Lorentz transformation and light aberration.] magnetic field orientations should be rotated by 90.
In Sgr A*,  polarized emission is present only towards the compact core, while none is observed from the mini-spiral.
In M87, the EVPA distribution appears  quite smooth along the jet, with no evident large fluctuations of the EVPAs  in nearby regions, except between Knots A and B.
For a negligible RM along the jet, one can infer that the magnetic field  orientation is first parallel to the jet axis, then in Knot A it changes direction (tending to be perpendicular to the jet), and then turns back to be parallel in Knot B, and finally becomes perpendicular to the jet axis further downstream (Knot C).
This behaviour  can be explained if Knot A is a standing or recollimation shock:
if multiple standing shocks with different magnetic field configurations  form along the jet and the latter is  threaded with a helical magnetic field, its helicity (or magnetic pitch) would be different before and after the  shock  owing to a different radial dependence of the poloidal and toroidal components of the magnetic field <cit.>. The EVPA distribution is also in good agreement with the polarization characteristics derived from observations at centimeter wavelengths with the VLA <cit.>.
We nevertheless explicitly  note that only the polarisation within the inner third of the primary beam is guaranteed by the ALMA observatory. Since we focus on the polarization properties in the core, the analysis presented in this paper is not affected by this systematics.

§.§.§ Rotation measure

Measuring the EVPA for each SPW (i.e., at four different frequencies) enables us to estimate the  RM
in the 3 mm band (spanning a 16 GHz frequency range of 85–101 GHz) and in the 1.3 mm band (spanning a 18 GHz frequency range of 212–230 GHz), respectively.
In the simplest assumption that the Faraday rotation is caused by a single external Faraday screen (i.e., it occurs outside of the plasma responsible for the polarized emission), a
linear dependence is expected between the EVPA and the wavelength squared.
In particular, we fit the RM and the mean-wavelength ($\bar{\lambda}$) EVPA ($\bar{\chi}$) following the relation:

\begin{equation}
\chi = \bar{\chi} + RM (\lambda^2 - \bar{\lambda}^2) \,,
\label{eq:rm}
\end{equation}

where $\chi$ is the observed EVPA at  wavelength $\lambda$ and $\bar{\chi}$ is the EVPA at wavelength $\bar{\lambda}$. The EVPA extrapolated to zero wavelength (assuming that the $\lambda^2$ relation holds) is:

\begin{equation}
\chi_{0} = \bar{\chi} - RM \bar{\lambda}^2 \,.
\label{eq:chi0}
\end{equation}

The RM fitting  is done using a weighted least-squares method of $\chi$ against $\lambda^2$.

The $\bar{\chi}, \ \chi_{0}$, and the fitted RM values are reported in the sixth, seventh, and eighth    columns of Tables <ref> and <ref>, respectively.

The EVPA uncertainties quoted in Tables <ref>,  <ref>, <ref>,  <ref>,   <ref>, are typically dominated by the systematic leakage of 0.03% of Stokes $I$ into Stokes $Q$ and $U$.
At 1.3 mm, this results in estimated errors between 0.06 for the most strongly polarized   source (3C279) and 0.8 for the weakest  source (J0132-1654), with most sources in the range $0.1$– $0.4$. These EVPA uncertainties imply RM propagated errors between $0.4 \times 10^4$  and $6 \times 10^4$ , with most sources in the range $(1-3) \times 10^4$ .
Similarly, at 3 mm we find  EVPA uncertainties of 0.07–1.4, with a typical value of 0.2, and RM uncertainties in the range $(0.06-1.0) \times 10^4$ , with a typical value of $\sim 0.13 \times 10^4$ .

In Appendix <ref>, we present plots of the measured EVPAs at the four ALMA SPWs and their RM fitted models as a function of $\lambda^2$ (displayed  in  Figures <ref>,  <ref>, <ref>, and <ref>)
we discuss the  magnitude of thermal and  systematic errors in the  RM analysis.

§.§.§ Bandwidth depolarization

In the presence of high RM,
the large EVPA rotation within the observing frequency bandwidth will decrease the measured fractional polarization owing to Faraday frequency or “bandwidth" depolarization, which depends on the observing frequency band. The RM values inferred in this study (e.g., Table <ref>) introduce an EVPA rotation of less than one degree within each 2 GHz spectral window, indicating that the bandwidth depolarization in these data should be very low ($<$0.005%). However, if there is an internal component of Faraday rotation (i.e., the emitting plasma is itself causing the RM), there will be much higher frequency-dependent (de)polarization effects (the “differential" Faraday rotation), which will be related to the structure of the Faraday depth across the source <cit.>.

We have modeled the frequency dependence of LP using a simple linear model:

\begin{equation}
m = \bar{m} + m'(\nu - \bar{\nu}) \,,
\label{eq:mnu}
\end{equation}

$m$ is the observed LP at frequency $\nu$,
$\bar{m}$ is the LP at the mean frequency $\bar{\nu}$, and $m'$ is the change of LP per unit frequency (in GHz$^{-1}$). Given the relatively narrow fractional bandwidth ($\lesssim$2 GHz), the linear approximation given in Eq. <ref> should suffice to model the frequency depolarization (multifrequency broadband single-dish studies fit more complex models; see for example  ).
We have fitted the values of $m'$ from a least-squares fit of Eq. <ref> to all sources and epochs, using
LP estimates for each spectral window from Table <ref>.
We show  the fitting results for selected sources in Fig. <ref> (lower panels). There are clear detections of $m'$ for 3C279, Sgr A*, and M87; these detections also differ between epochs.

Such complex time-dependent frequency effects in the polarization intensity may be indicative of an internal contribution to the Faraday effects observed at mm wavelengths.

§.§.§ Circular polarization

Measuring Stokes V provides, in principle, a direct estimate of the fractional CP as $V/I$.
In practice, the polarization calibration for ALMA data in  is done by solving the polarization equations in the linear approximation, where parallel-hands and cross-hands visibilities are expressed as a linear function of $I$, $Q$, $U$, while it is assumed $V=0$ <cit.>.
A non-negligible Stokes $V$ in the polarization calibrator will introduce a spurious instrumental Stokes $V$ into the visibilities of all the other sources.
Moreover, such a Stokes $V$ introduces a bias in the estimate of the cross-polarization phase, $\beta$, at the reference antenna (see Appendix <ref>), which translates into a leakage-like effect in the polconverted VLBI visibilities <cit.>.
The magnitude of such a bias may depend on the fractional CP of the polarization calibrator, the parallactic-angle coverage of the calibrator, and the specifics of the calibration algorithm.
In Appendix <ref> we attempt to estimate such a spurious contribution to Stokes $V$ by computing the cross-hands visibilities of the polarization calibrator  as a function of parallactic angle (see Figures <ref> and <ref>).
This information can then be used to assess Stokes $V$ and CP for all sources in all days (reported in Tables <ref> and <ref> for  GMVA and EHT sources, respectively).

We stress two main points here.
First, the reconstructed Stokes $V$ values of the polarization calibrators are non-negligible and are therefore expected to introduce a residual instrumental X–Y phase difference in all other sources, after QA2 calibration.
This can be  seen in the dependence of the reconstructed Stokes $V$ with feed angle in almost all the observed sources (displayed in Figure <ref>).
The  estimated X–Y residual phase offsets are of the order of $0.5^{\circ}$, but they can be as high as $2^{\circ}$ (e.g., on Apr 5). These values would translate to a (purely imaginary) leakage term of the order of a few % in the polconverted VLBI visibilities.

The second point is that there is a significant variation in the estimated values of reconstructed stokes $V$  across the observing week.
In particular, on Apr 5, 3C279 shows a much higher value, indicating either an intrinsic change in the source, or systematic errors induced by either the instrument or the calibration.
In either case, this anomalously large Stokes $V$ in the polarization calibrator introduces a large X–Y phase difference in all other sources.
This can be seen in the strong dependence of reconstructed Stokes $V$  on feed angle for sources OJ287 and 4C 01.28 (displayed in Figure <ref>, upper left panel) and in their relatively high Stokes $V$ when compared to the following days (see Table <ref>).
Besides the anomalous value in Apr 5, it is interesting to note that the data depart from the sinusoidal model described by Eq. <ref>, for observations far from transit, especially on Apr 11. These deviations may be related to other instrumental effects which however we are not able to precisely quantify.
For these reasons, we cannot precisely estimate the magnitude of the true CP  fractions for the observed sources (see Appendix <ref> for details).
Nevertheless, our analysis still enables us to obtain order-of-magnitude values of CP.
In particular, excluding the anomalous Apr 5, we report CP =[–1.0,–1.5] %  in Sgr A*,
CP$\sim$ 0.3% in M87, and possibly a lower CP level ($\sim 0.1-0.2$%) in a few other AGNs (3C273, OJ287, 4C 01.28, J0132-1654, J0006-0623; see Table <ref>).
In the 3 mm band, we do not detect appreciable CP above 0.1%, except for 4C 09.57 (–0.34%), J0510+1800 (–0.14%) and 3C273 (0.14%).

We however note that the official accuracy of CP guaranteed by the ALMA observatory is $<$ 0.6% ($1\sigma$) or 1.8% ($3\sigma$), and therefore all of these CP measurements should be regarded as tentative detections.

§ RESULTS

§.§ AGN

We observed a dozen AGN, eight at 3 mm and ten at 1.3 mm (with six observed in both bands), in addition to M87.
Following the  most prevalent  classification scheme found in the literature <cit.>,
our sample includes three radio galaxies (M87, NGC 1052, Cen A),
three BL Lacs (OJ 287, J0006-0623, 4C 09.57), and
seven additional QSOs (3C 273, 3C 279, NRAO 530, 4C 01.28,  J1924-2914, J0132-1654, and J0510+1800).
Following the standard definition of blazar (i.e. an AGN with a relativistic jet nearly directed towards the L.O.S.), we can further combine the last two categories into seven blazars (3C 279, OJ 287, J1924-2914,  4C 01.28, 4C 09.57, J0006-0623, J0510+1800) and three additional QSOs (3C 273, NRAO 530, J0132-1654).
The observed radio galaxies have a core that is considered as a LLAGN <cit.>.

Their polarimetric quantities at 3 mm and  1.3 mm  are reported in Tables <ref> and <ref>, respectively, and displayed  in Figure <ref>.
Overall, we find LP fractions in the range 0.1–15% (with $SNR \sim 3-500\sigma$) and RM in the range $10^{3.3}-10^{5.5}$ rad/m$^2$ (with $SNR \sim 3-50\sigma$), in line with previous studies at mm-wavelengths with single-dish telescopes <cit.> and interferometers  <cit.>.
We also constrain CP to $<$0.3% in all the observed AGN, consistent with previous single-dish  <cit.> and VLBI <cit.> studies, suggesting that at mm-wavelengths  AGN are  not strongly circularly polarized and/or that Faraday conversion of the linearly polarized synchrotron emission is not an efficient process <cit.>.

In Appendix <ref>, we also report maps  of all the AGN targets observed at 1.3 mm (Figures <ref>, <ref>, <ref>), and  at 3 mm (Figure <ref>), showcasing their arcsecond-structure at mm wavelengths.

In the rest of this section, we briefly comment on the properties of selected AGN.

3C 279
3C 279 is a bright and highly magnetized gamma-ray emitting blazar, whose jet is inclined at a very small viewing angle ($\lesssim3$). At its distance  (z=0.5362), 1 arcsecond subtends 6.5 kpc. 3C 279 was observed on four days at 1.3 mm and one day at 3 mm.
It is remarkably highly polarized both at 1.3 mm and 3 mm.
At 1.3 mm, LP varies from 13.2% on Apr 5 to 14.9% in Apr 11, while the EVPA goes from 45 down to 40.
At 3 mm, LP is slightly lower ($\sim12.9$%) and the EVPA is 44.

While at 1.3 mm we can only place a $1\sigma$ upper limit of $<$5000 rad/m$^2$,
at 3 mm we measure a RM$=1790 \pm 460$ rad/m$^2$ (with a $\sim4\sigma$ significance).

<cit.> used the Korean VLBI Network to measure the LP at 13, 7, and 3.5 mm, finding  RM values in the range  -650  to -2700 , which appear to scale as a function of wavelength as $\lambda^{-2.2}$.
These VLBI measurements are not inconsistent with  our 3 mm measurement and our upper limits at 1.3 mm, but more accurate measurements at higher frequencies are needed to confirm an  increase of the RM with frequency.

The total intensity images at 1.3 mm reveal, besides the bright core, a jet-like feature extending approximately 5 towards south-west (SW)  (Fig. <ref>); such a feature is not discernible in the lower-resolution 3 mm image (Fig. <ref>).
The  jet-like feature is oriented at approximately 40, i.e. is roughly aligned with the EVPA in the core.
Ultra-high resolution images with the EHT reveal a jet component
approximately along the same PA but on angular scales $10^5$  times
smaller <cit.>.

3C 273
3C 273 was the first discovered quasar <cit.>, and is one of the closest (z=0.158, 1 arcsec = 2.8 kpc)  and brightest radio-loud quasars.
3C 273 was observed both at 1.3 mm and 3 mm (2 days apart).
Total intensity and LP are higher in the lower frequency band: F=9.9 Jy and LP=4.0% (at 3 mm) vs. F=7.6 Jy and LP=2.4% (1.3 mm).
We estimate a RM = $(2.52 \pm 0.27) \times 10^5$ rad/m$^2$ at 1.3 mm, confirming the high RM revealed in previous
ALMA observations (conducted in Dec 2016 with 08 angular resolution) by <cit.> who report LP=1.8% and a  (twice as large) RM$=(5.0 \pm 0.3) \times 10^5$ rad/m$^2$.
We also report  for the first time a RM measurement at 3 mm,  RM = $(-0.60 \pm 0.14) \times 10^4$ rad/m$^2$, about 40 times lower and with opposite sign with respect to  the higher frequency band.
The $\chi_0$ changes from $-82\pm3$ at 1.3 mm to $-41.9\pm0.8$ at 3 mm. These large differences may be explained with opacity effects <cit.>.
The EVPAs  measured at 3 mm and 1.3 mm are in excellent agreement with predictions from the AMAPOLA survey (which however over-predicts LP$\sim$3.5% at 1.3 mm; see Fig. <ref>).

The total intensity images both at 1.3 mm and 3 mm display, besides  the bright core, a bright, one-sided jet extending approximately 20 (54 kpc) to the SW.
In the higher resolution 1.3 mm image (Fig. <ref>), the bright component of the jet is narrow and nearly straight,
starts  at a separation of  $\sim$10 from the core and has a length of  $\sim$10. We also detect (at the 3$\sigma$ level) two weak components of the inner jet (within $\sim$10 from the core) joining the bright nucleus to the outer jet.  The  jet structure is qualitatively similar to previous $\lambda$ cm images made with the VLA at several frequencies between 1.3 and 43 GHz <cit.>, where the   outer jet appears highly linearly polarized[ <cit.> report an LP as high as 55% in their at 15 GHz map along the jet boundaries (although in the central regions LP is much lower).].
We do not detect LP in the jet feature.

OJ 287
The bright blazar OJ 287 (z=0.306,  1 arcsec = 4.7 kpc) is among the best candidates for hosting a compact supermassive binary black hole <cit.>.
OJ287 was observed on three days at 1.3 mm and one day at 3 mm[These ALMA observations of OJ 287 in April 2017 were preceded by a major X-ray–optical outburst in late 2016 to early 2017 <cit.>.].
OJ287 is one of the most highly polarized targets both at 1.3 mm (LP$\sim 7-9$%) and 3 mm (LP = 8.8%). LP drops from 9% on Apr 5 down to 7% on Apr 10, while
the EVPA is stable around [-59.6,-61.8]  at  1.3 mm and -70  at  3 mm.
The LP variation and stable EVPA are consistent with the historical trends derived from the AMAPOLA survey (see Fig. <ref>).
Its flux density is also stable.
At 1.3mm, the EVPA either does not follow a $\lambda^2$-law  (Apr 5 and 11) or the formal fit is consistent with RM = 0 (Apr 10).
Although we do not have a RM detection at 1.3 mm, we  measure a RM $=3050 \pm 620$ rad/m$^2$ at 3 mm.
A 30 years monitoring of  the radio jet in OJ287 has revealed that its (sky-projected) PA varies  both at cm and mm wavelengths and follows the modulations of the EVPA at optical wavelengths <cit.>.
The observed EVPA/jet-PA trend can be explained with a jet precessing model from the binary black hole which successfully predict an optical EVPA = –66.5 in 2017 (Dey et al., submitted), consistent with actual measurements from optical  polarimetric observations during 2016/17  <cit.>
and close to the EVPA measured at 3 mm and 1.3 mm with ALMA.

NRAO 530
J1733-1304 (alias NRAO 530) is a highly variable QSO (at z = 0.902; 1 arcsec = 8 kpc) that exhibits strong gamma-ray flares.
It was observed on two consecutive days at 1.3 mm and one day at 3 mm.
It is linearly polarized at a $\sim$2.4% level at 1.3 mm but only 0.9%  at 3 mm.
The EVPA goes from $\sim51$ at 1.3 mm to 39 at 3 mm, while $\chi_0$ is stable around 51–52.
At  3 mm, we estimate RM = $-0.21\pm0.06 \times 10^5$ rad/m$^2$ at a $3.6\sigma$ significance, which is comparable to the inter-band RM
between 1 and 3 mm ($-0.33 \times 10^5$ rad/m$^2$).
These RM values  are in agreement with those reported by <cit.> at 1.3 mm.

The arcsecond-scale  structure at 1.3 mm is dominated by a compact core with a second weaker component at a separation of approximately 10 from the core towards west (Fig. <ref>). At 3 mm, there is another
feature in opposite direction (to the east), which could be a counter jet component (Fig. <ref>).
This geometry is apparently inconsistent with the north-south elongation of the jet revealed on scales $<100$ pc by recent VLBI multi-frequency  (22, 43 and 86 GHz) imaging  <cit.>, although the Boston University Blazar monitoring program[<https://www.bu.edu/blazars/VLBA_GLAST/1730.html>] conducted with the  VLBA at 7 mm has revealed significant changes in the jet position angle over the years, and possibly jet bending.

is  a  radio-loud  blazar at z=0.352 (1 arcsec = 5.1 kpc),  which shows  strong  variability  from  radio  to  X-ray.
It was observed on three days at 1.3 mm and one day at 3 mm.
J1924-2914  appears strongly polarized with LP varying from 6.1% (on Apr 6) to 4.9% (on Apr 11) at 1.3 mm, and LP=4.8% (on Apr 4) at 3 mm.
The EVPA is stable around [-49.2,-51.8]  at  1.3 mm and -46.4  at  3 mm.
We report a RM $\sim ([0.3-0.4] \pm 0.1) \times 10^5$ rad/m$^2$ at 1.3 mm and a $ 3\sigma$ upper limit of 3600   at 3 mm (approximately an order of magnitude lower).
<cit.> report a higher RM value of $(-0.71\pm0.06)\times10^5$  at 1.3 mm from ALMA observations carried out in August 2016, when the source LP was considerably lower ($\lesssim2\%$).
The  AMAPOLA monitoring revealed a considerable variation in the source EVPA  during Mar–Dec 2016[<www.alma.cl/$\sim$skameno/AMAPOLA/J1924-2914.flux.html>], likely due to a period of low LP. We therefore ascribe the difference with the <cit.> measurement  to source variability.

J1924-2914  is completely unresolved on arcsecond scales both at 1.3 mm and 3 mm (see Figures <ref> and <ref>), a result consistent with images at cm-wavelengths made with the VLA   <cit.>.

4C 01.28
J1058+0133 (alias 4C 01.28) is a blazar at z= 0.888 (1 arcsec = 8 kpc).
It was observed on three days at 1.3 mm and one day at 3 mm.
The source is  strongly polarized with a mean LP of 5.5% at 1.3 mm  and 4.4% at 3 mm.
At 1.3 mm, the LP  varies by $<$15% while the EVPA changes from $\sim-23$ (Apr 5) to $\sim-15$ (Apr 11); the EVPA at 3 mm, measured on Apr 2, is –32, apparently consistent with the trend at 1.3 mm.
Both the measured EVPA and LP values at 1.3 mm and 3 mm follow very closely the time evolution predicted in the AMAPOLA survey (see Fig. <ref>),
where the  LP and  EVPA  follow a  trend parallel to the Stokes $I$ evolution.
On Apr 11, we tentatively detect  RM $\sim (0.33\pm0.13)  \times 10^5$  at the $\sim 3\sigma$ level;  we however caution that on Apr 5 and 10 the EVPAs  do not follow the $\lambda^2$ trend  (Fig. <ref>), and  we do not have a RM detection at 3 mm (with a $3\sigma$ upper limit of 3600 ).

Cen A
Centaurus A (Cen A) is the closest radio-loud AGN (at a distance of 3.8 Mpc, 1 arcsec = 18 pc).
Although it is a bright mm source (with F=5.7 Jy), it is unpolarized  at 1.3 mm (with a $3\sigma$ LP upper limit of 0.09%).
We find a spectral index of –0.2 in the central core, consistent with a flat spectrum,
as also measured between 350 and 698 GHz with (non-simultaneous) ALMA observations <cit.>.

The total intensity images reveal a diffuse emission component around the central bright core, extending   across 12 and mostly elongated north-south, and two additional compact components towards north-east (NE) separated by roughly 14 and 18 from the central core and aligned  at P.A.$\sim$50 (see Fig. <ref>, bottom-right panel).
The first component could be associated with the inner circumnuclear disk, mapped in CO with the SMA  <cit.> and ALMA <cit.>, and may  indicate the presence of a dusty torus.
The two additional components correspond to two knots of the northern lobe of the relativistic jet,  labelled as A1 (inner) and A2 (outer) in a VLA study by <cit.>; no portion of the southern jet is seen,  consistent with previous observations <cit.>.

NGC 1052
NGC 1052 is a  nearby (19.7 Mpc; 1 arcsec = 95 pc)  radio-galaxy  that showcases an exceptionally bright twin-jet  system with a large viewing angle close to 90 degrees <cit.>.
With  F$\sim$0.4 Jy and LP$<$0.15%, it is the  weakest mm source (along with J0132-1654) and the second least polarized AGN in  our sample.
The apparent discrepancy in flux-density and spectral index between Apr 6 and 7 is most likely a consequence of the low flux-density (below the threshold required by the  commissioned on-source phasing mode; see  <ref>) and the much poorer data quality on Apr 7, rather than  time-variability of the source.

Remaining AGN
J0006-0623  is the most highly polarized blazar (after 3C279) observed at 1.3 mm, with LP = 12.5%.
J0132-1654 is the weakest QSO observed at 1.3 mm ($F\sim$0.4 Jy) and has LP$\sim2$%.
The blazar J0510+1800 has an LP $\sim$ 4% at 3 mm and shows  indication of a  large RM ($\sim 0.27\times10^5\,$rad/m$^2$), although the EVPA distribution does not follow a $\lambda^2-$dependence (see Fig. <ref>, upper-right panel).

§.§ M87

We report the first unambiguous measurement of RM toward the M87 nucleus at mm wavelengths (Table <ref>; Figure <ref>, middle panels).
We measure
$(1.51\pm0.30) \times 10^5$  (with a $5\sigma$ significance) on Apr 6
and tentatively $(0.64\pm0.27) \times 10^5$  (with a $2.4\sigma$ significance) on Apr 5.
On the last two days we can only report best-fit values of
$(-0.24\pm0.23) \times 10^5$  (with a $3\sigma$ confidence level range $ [-0.93,0.45] $) on Apr 10 and
$(-0.39\pm0.24) \times 10^5$  (with a $3\sigma$ confidence level range $ [-1.11,0.33] $) on Apr 11.

Although we cannot determine precisely the RM value on all days, we can conclude that the RM appears to vary substantially across days and there is marginal evidence of sign reversal.

Before this study, the only RM measurement was done with the SMA at 230 GHz by <cit.>, who reported a best-fit RM = $ (-2.1\pm1.8) \times 10^5$   ($1\sigma$ uncertainty) and could therefore only provide an upper limit.

In order to better constrain the RM amplitude and its time variability, in addition to the 2017 VLBI observations (which are the focus of this paper),
we have also analysed the ALMA data acquired during the Apr 2018 VLBI campaign as well as additional ALMA archival polarimetric experiments (these are introduced in <ref>).
For two projects (2016.1.00415.S and 2017.1.00608.S) we produced fully-calibrated $uv$-files and then used the  flux extraction method with  to determine the M87 Stokes parameters.
For the remaining two projects (2013.1.01022.S and 2015.1.01170.S), we used the full-Stokes images released with QA2. Since these images do not include clean component models, we used the    method  to extract the Stokes parameters in the compact core directly in the images[Based on the analysis of the 2017 datasets, we have  assessed that
yields consistent polarimetric parameters with respect to  and  (see   <ref> and Appendix <ref>).].

Table <ref> reports the full list of ALMA observations,  project codes, and derived polarimetric parameters.
In total, we have collected data from three and eight different observations at 3 mm and 1.3 mm, respectively, spanning three years (from Sep 2015 to Sep 2018).
The main findings revealed by the analysis of the full dataset are the following:

* The total flux density is quite stable on a week timescale, varying by $\lesssim$5% in both Apr 2017 and Apr 2018, and exhibiting total excursion of about 15-20% across one year both at 1.3 mm (decreasing from Apr 2017 to Apr 2018) and 3 mm (increasing from Sep 2015 to Oct 2016).

* We detect LP $\sim$ 1.7–2.7% (2.3% mean; Apr 2017 – Apr 2018) at 1.3 mm and LP $\sim$ 1.3–2.4% (1.7% mean; Sep 2015 – Oct 2016) at 3 mm.

* The EVPA  distributions across the four ALMA SPWs clearly display  a $\lambda^2$ dependence, on specific dates, within both the 1.3 mm and 3 mm bands (e.g., see Figures <ref> and <ref>).

* The magnitude of the RM varies both at 3 mm (range $| \rm{RM} | =   [ 0.2-1.2 ] \times 10^5$ ) and 1.3 mm (range $| \rm{RM} | =   [ 1.5-4.1 ] \times 10^5$ , including $<3\sigma$ non-detections).

* The RM can either be positive or negative in both bands (with a preference for a negative sign), indicating that sign flips are present both at 3 mm and 1.3 mm.

* In Apr 2017, the RM magnitude appears to vary significantly (from non-detection  up to 1.5 $\times$ $10^5$ ) in just 4–5 days.

* In Apr 2017, $\chi_0$  varies substantially across a week, being
[$-14.6\pm2.8,\ -23.6\pm3.1,\ 2.5\pm2.5,\ 3.5\pm2.5$] in Apr 5, 6, 10, and 11, respectively. Therefore, although the EVPA at 1.3 mm changes only by $\sim$+8 during the observing week, the $\chi_0$ varies by -9 in the first two days, and +27 between the second day and the last two days.

In Apr 2018 $\chi_0$ appears instead to be consistently around 68.4–70.6[The change of about +10 in the EVPA at 1.3 mm between Apr 21 and Apr 25 2018 can be completely explained with a decrease in RM $\sim - 1 \times 10^5$ .].
The  $\chi_0$ derived from  the three 3 mm experiments (Sep, Nov 2015 and Oct 2016) spans a range from $\sim$4 to 107 (see Fig. <ref> for a summary plot of RM+$\chi_0$ in all the available M87 observations).

* The EVPAs measured  at 1.3 mm in the 2017 campaign ($\sim [-8,0]$) are significantly different to the ones measured in the 2018 campaign ($\sim [26,36]$), which are instead consistent with the ones measured in 2015–2016  at 3 mm ($\sim [21,33]$).

* We find hints of CP at 1.3 mm at the $[-0.3 \pm 0.6,-0.4\pm 0.6]$% level, but these should be regarded as tentative measurements (see also  Appendix <ref> for caveats on the CP estimates).

We will interpret these findings in Section <ref>.

M87 EVPA as a function of $\lambda^2$ observed in multiple epochs  at 3 mm (from Sep 2015 to Nov 2016) and 1.3 mm  (from Apr 2017 to Apr 2018).
Each grey line is a linear fit to the EVPAs measured at the four ALMA Band 3 and Band 6 SPW, yielding the RM in each epoch, and the extrapolated intercept at the Y-axis is $\chi_0$. Note the large offset in  $\chi_0$ between the 3 mm and 1.3 mm bands.

§.§ Sgr A*

In this section, we analyse the polarimetric properties of Sgr A* and its variability on a week timescale based on  the ALMA observations at 1.3 mm and 3 mm.

We measured LP between 6.9% and 7.5% across one week at 1.3 mm (Table <ref>).
These values are broadly consistent with historic  measurements using BIMA on several epochs in the time span 2002–2004 at 227 GHz <cit.>,
SMA on several days in Jun–Jul 2005
and more recently  with ALMA in Mar–Aug 2016 at 225 GHz
Besides observations at 1.3 mm, LP variability  has been reported also at 3.5 mm with BIMA <cit.> and at 0.85 mm with the SMA <cit.>.
All together, these measurements imply significant time-variability of LP across timescales of hours/days to months/years.

While at 1.3 mm LP $\sim$ 7%, at 3 mm we detect  LP $\lesssim$ 1% (Table <ref>).
It is interesting to note that the LP fraction increases from $\sim$ 0.5% at $\sim$ 86 GHz (our SPW=0,1) up to $\sim$ 1% at $\sim$ 100 GHz (our SPW=2,3; see Table <ref>).
This trend is  consistent with earlier measurements  at 22 GHz and 43 GHz with the VLA, and at 86 GHz and 112–115 GHz with BIMA, yielding upper limits of LP$\sim$ 0.2, 0.4%, 1% <cit.>, and 1.8% <cit.>, respectively <cit.>.

We report a mean RM of $-4.2 \times10^5$   at 1.3 mm with a significance of $\sim 50\sigma$ (Table <ref>; Figure <ref>, upper second to fourth panels), consistent with measurements over the past 15 years since the first measurements with BIMA+JCMT <cit.>, BIMA+JCMT+SMA <cit.>, and the SMA alone <cit.>[Both <cit.> and <cit.> used non-simultaneous EVPA measurements in the frequency range 150–400 GHz and 83–400 GHz, respectively. <cit.> determined for the first time the RM  comparing EVPAs
measured simultaneously at each (1.3 and 0.85 mm) band.].
Across the observing week,  we see a change in RM from $-4.84 \pm 0.1\times10^5$   (on Apr 6) to $-3.28 \pm 0.09 \times10^5$   (on Apr 11), corresponding to a change of $\sim -1.5 \times10^5$   ($\sim$30%), detected with a significance of $\sim 20\sigma$.
This RM change  can completely explain the EVPA variation from  –65.8$\pm$0.1 to  –49.3$\pm$0.1 (or a $\sim$16 change across 5 days), given the consistency in $\chi_0$ between Apr 6  and Apr 11 ($\sim -14.7$$\pm1.0$; see Table <ref>).

<cit.> find a comparable dispersion based on six measurements in the time period Jun–Jul 2005 ($\Delta |{\rm RM}|= 1.3  \times 10^5$   excluding their most discrepant point, or $\Delta |{\rm RM}|= 3.8  \times 10^5$   including all 6 measurements spanning almost 2 months).
<cit.> find an even larger  $\Delta |{\rm RM}|=-4.9 \times10^5$   across 5 months; they also report intra-day variability in a similar range on timescales of several hours.

Variations in RM appear to be coupled with LP fraction: the lower the polarization flux density, the higher the absolute value of the RM. In particular, we find $\Delta {\rm LP} \sim +5\%$ ($\Delta {\rm RM}\sim -9\%$) and $\Delta {\rm LP} \sim +9$% ($\Delta {\rm RM}\sim -32\%$) in Apr 7 and 11, respectively, with respect to Apr 6.
This can be understood if a larger RM scrambles more effectively the polarization vector fields resulting in lower net polarization.
Although with only three data points we cannot draw a statistically  significant conclusion, we note that the same trend was also seen by <cit.> on shorter (intra-day) timescales.

We report for the first time a measurement of  RM at 3 mm, with a magnitude of $(-2.1\pm0.1) \times10^5$  (Table <ref>; Figure <ref>, upper-left panel).
The RM magnitude at 3 mm (measured on Apr 3) is  a factor of 2.3 (2.1) smaller  than the RM value measured at 1.3 mm on Apr 6 (Apr 7).
Furthermore, we note a large offset in  $\chi_0$ between the 3 mm (+135  or –45 for a full 180 wrap) and the 1.3 mm bands ($\sim [-14.7,-18.8]$), which is unlikely a consequence of time variability  (given the $\chi_0$ consistency  on Apr 6–11).
The comparison of  RM and $\chi_0$ in the two frequency bands (showcased in Fig. <ref>) indicates the presence of both Faraday and intrinsic changes of the source.
We will provide an interpretation of the differences observed between the two frequency bands in  <ref>.

Sgr A* EVPA as a function of $\lambda^2$ observed in 2017 at 3 mm (Apr 3) and 1.3 mm (Apr 6, 7, 11).