Draft version September 25, 2020
Typeset using L
A
T
E
X
twocolumn
style in AASTeX63
Monitoring the Morphology of M87
∗
in 2009–2017 with the Event Horizon Telescope
Maciek Wielgus,
1, 2
Kazunori Akiyama,
3, 4, 5, 1
Lindy Blackburn,
1, 2
Chi-kwan Chan,
6, 7
Jason Dexter,
8
Sheperd S. Doeleman,
1, 2
Vincent L. Fish,
4
Sara Issaoun,
9
Michael D. Johnson,
1, 2
Thomas P. Krichbaum,
10
Ru-Sen Lu (
路
如
森
),
11, 10
Dominic W. Pesce,
1, 2
George N. Wong,
12, 13
Geoffrey C. Bower,
14
Avery E. Broderick,
15, 16, 17
Andrew Chael,
18, 19
Koushik Chatterjee,
20
Charles F. Gammie,
12, 21
Boris Georgiev,
16, 17
Kazuhiro Hada,
22, 23
Laurent Loinard,
24, 25
Sera Markoff,
20, 26
Daniel P. Marrone,
6
Richard Plambeck,
27
Jonathan Weintroub,
1, 2
Matthew Dexter,
27
David H. E. MacMahon,
27
Melvyn Wright,
27
Antxon Alberdi,
28
Walter Alef,
10
Keiichi Asada,
29
Rebecca Azulay,
30, 31, 10
Anne-Kathrin Baczko,
10
David Ball,
6
Mislav Baloković,
1, 2
Enrico Barausse,
32, 33
John Barrett,
4
Dan Bintley,
34
Wilfred Boland,
35
Katherine L. Bouman,
1, 2, 36
Michael Bremer,
37
Christiaan D. Brinkerink,
9
Roger Brissenden,
1, 2
Silke Britzen,
10
Dominique Broguiere,
37
Thomas Bronzwaer,
9
Do-Young Byun,
38, 39
John E. Carlstrom,
40, 41, 42, 43
Shami Chatterjee,
44
Ming-Tang Chen,
14
Yongjun Chen (
陈
永
军
),
11, 45
Ilje Cho,
38, 39
Pierre Christian,
6, 2
John E. Conway,
46
James M. Cordes,
44
Geoffrey B. Crew,
4
Yuzhu Cui,
22, 23
Jordy Davelaar,
9
Mariafelicia De Laurentis,
47, 48, 49
Roger Deane,
50, 51
Jessica Dempsey,
34
Gregory Desvignes,
52
Sergio A. Dzib,
10
Ralph P. Eatough,
10
Heino Falcke,
9
Ed Fomalont,
3
Raquel Fraga-Encinas,
9
Per Friberg,
34
Christian M. Fromm,
48
Peter Galison,
1, 53, 54
Roberto García,
37
Olivier Gentaz,
37
Ciriaco Goddi,
9, 55
Roman Gold,
56, 15
José L. Gómez,
28
Arturo I. Gómez-Ruiz,
57, 58
Minfeng Gu (
顾
敏
峰
),
11, 59
Mark Gurwell,
2
Michael H. Hecht,
4
Ronald Hesper,
60
Luis C. Ho (
何
子
山
),
61, 62
Paul Ho,
29
Mareki Honma,
22, 23
Chih-Wei L. Huang,
29
Lei Huang (
黄
磊
),
11, 59
David H. Hughes,
63
Makoto Inoue,
29
David J. James,
64
Buell T. Jannuzi,
6
Michael Janssen,
9
Britton Jeter,
16, 17
Wu Jiang (
江
悟
),
11
Alejandra Jimenez-Rosales,
65
Svetlana Jorstad,
66, 67
Taehyun Jung,
38, 39
Mansour Karami,
15, 16
Ramesh Karuppusamy,
10
Tomohisa Kawashima,
5
Garrett K. Keating,
2
Mark Kettenis,
68
Jae-Young Kim,
10
Junhan Kim,
6, 36
Jongsoo Kim,
38
Motoki Kino,
5, 69
Jun Yi Koay,
29
Patrick M. Koch,
29
Shoko Koyama,
29
Michael Kramer,
10
Carsten Kramer,
37
Cheng-Yu Kuo,
70
Tod R. Lauer,
71
Sang-Sung Lee,
38
Yan-Rong Li (
李
彦
荣
),
72
Zhiyuan Li (
李
志
远
),
73, 74
Michael Lindqvist,
46
Rocco Lico,
10
Kuo Liu,
10
Elisabetta Liuzzo,
75
Wen-Ping Lo,
29, 76
Andrei P. Lobanov,
10
Colin Lonsdale,
4
Nicholas R. MacDonald,
10
Jirong Mao (
毛
基
荣
),
77, 78, 79
Nicola Marchili,
75, 10
Alan P. Marscher,
66
Iván Martí-Vidal,
30, 31
Satoki Matsushita,
29
Lynn D. Matthews,
4
Lia Medeiros,
80, 6
Karl M. Menten,
10
Yosuke Mizuno,
48, 81
Izumi Mizuno,
34
James M. Moran,
1, 2
Kotaro Moriyama,
4, 22
Monika Moscibrodzka,
9
Cornelia Müller,
10, 9
Gibwa Musoke,
20, 9
Hiroshi Nagai,
5, 23
Neil M. Nagar,
82
Masanori Nakamura,
29
Ramesh Narayan,
1, 2
Gopal Narayanan,
83
Iniyan Natarajan,
51
Antonios Nathanail,
48
Roberto Neri,
37
Chunchong Ni,
16, 17
Aristeidis Noutsos,
10
Hiroki Okino,
22, 84
Héctor Olivares,
48
Gisela N. Ortiz-León,
10
Tomoaki Oyama,
22
Feryal Özel,
6
Daniel C. M. Palumbo,
1, 2
Jongho Park,
29
Nimesh Patel,
2
Ue-Li Pen,
15, 85, 86, 87
Vincent Piétu,
37
Aleksandar PopStefanija,
83
Oliver Porth,
20, 48
Ben Prather,
12
Jorge A. Preciado-López,
15
Dimitrios Psaltis,
6
Hung-Yi Pu,
15, 88, 29
Venkatessh Ramakrishnan,
82
Ramprasad Rao,
14
Mark G. Rawlings,
34
Alexander W. Raymond,
1, 2
Luciano Rezzolla,
48, 89
Bart Ripperda,
90, 91
Freek Roelofs,
9
Alan Rogers,
4
Eduardo Ros,
10
Mel Rose,
6
Arash Roshanineshat,
6
Helge Rottmann,
10
Alan L. Roy,
10
Chet Ruszczyk,
4
Benjamin R. Ryan,
13, 92
Kazi L. J. Rygl,
75
Salvador Sánchez,
93
David Sánchez-Arguelles,
63, 58
Mahito Sasada,
22, 94
Tuomas Savolainen,
95, 96, 10
F. Peter Schloerb,
83
Karl-Friedrich Schuster,
37
Lijing Shao,
10, 62
Zhiqiang Shen (
沈
志强
),
11, 45
Des Small,
68
Bong Won Sohn,
38, 39, 97
Jason SooHoo,
4
Fumie Tazaki,
22
Paul Tiede,
16, 17
Remo P. J. Tilanus,
9, 55, 98, 6
Michael Titus,
4
Kenji Toma,
99, 100
Pablo Torne,
10, 93
Tyler Trent,
6
Efthalia Traianou,
10
Sascha Trippe,
101
Shuichiro Tsuda,
22
Ilse van Bemmel,
68
Huib Jan van Langevelde,
68, 102
Daniel R. van Rossum,
9
Jan Wagner,
10
John Wardle,
103
Derek Ward-Thompson,
104
Norbert Wex,
10
Robert Wharton,
10
Qingwen Wu (
吴
庆
文
),
105
Doosoo Yoon,
20
André Young,
9
Ken Young,
2
maciek.wielgus@gmail.com
arXiv:2009.11842v1 [astro-ph.HE] 24 Sep 2020
2
Ziri Younsi,
106, 48
Feng Yuan (
袁
峰
),
11, 59, 107
Ye-Fei Yuan (
袁
业
飞
),
108
J. Anton Zensus,
10
Guangyao Zhao,
38
Shan-Shan Zhao,
9, 73
and Ziyan Zhu
54
1
Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
2
Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
3
National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA
4
Massachusetts Institute of Technology Haystack Observatory, 99 Millstone Road, Westford, MA 01886, USA
5
National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
6
Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
7
Data Science Institute, University of Arizona, 1230 N. Cherry Ave., Tucson, AZ 85721, USA
8
JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309, USA
9
Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box
9010, 6500 GL Nijmegen, The Netherlands
10
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
11
Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, People’s Republic of China
12
Department of Physics, University of Illinois, 1110 West Green St, Urbana, IL 61801, USA
13
CCS-2, Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545, USA
14
Institute of Astronomy and Astrophysics, Academia Sinica, 645 N. A’ohoku Place, Hilo, HI 96720, USA
15
Perimeter Institute for Theoretical Physics, 31 Caroline Street North, Waterloo, ON, N2L 2Y5, Canada
16
Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada
17
Waterloo Centre for Astrophysics, University of Waterloo, Waterloo, ON N2L 3G1 Canada
18
Princeton Center for Theoretical Science, Jadwin Hall, Princeton University, Princeton, NJ 08544, USA
19
NASA Hubble Fellowship Program, Einstein Fellow
20
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
21
Department of Astronomy, University of Illinois at Urbana-Champaign, 1002 West Green Street, Urbana, IL 61801, USA
22
Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Mizusawa, Oshu, Iwate 023-0861, Japan
23
Department of Astronomical Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo
181-8588, Japan
24
Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Morelia 58089, México
25
Instituto de Astronomía, Universidad Nacional Autónoma de México, CdMx 04510, México
26
Gravitation Astroparticle Physics Amsterdam (GRAPPA) Institute, University of Amsterdam, Science Park 904, 1098 XH Amsterdam,
The Netherlands
27
Radio Astronomy Laboratory, University of California, Berkeley, CA 94720, USA
28
Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
29
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.
30
Departament d’Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, E-46100 Burjassot, València, Spain
31
Observatori Astronòmic, Universitat de València, C. Catedrático José Beltrán 2, E-46980 Paterna, València, Spain
32
SISSA, Via Bonomea 265, 34136 Trieste, Italy and INFN Sezione di Trieste
33
IFPU - Institute for Fundamental Physics of the Universe, Via Beirut 2, 34014 Trieste, Italy
34
East Asian Observatory, 660 N. A’ohoku Place, Hilo, HI 96720, USA
35
Nederlandse Onderzoekschool voor Astronomie (NOVA), PO Box 9513, 2300 RA Leiden, The Netherlands
36
California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA
37
Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint Martin d’Hères, France
38
Korea Astronomy and Space Science Institute, Daedeok-daero 776, Yuseong-gu, Daejeon 34055, Republic of Korea
39
University of Science and Technology, Gajeong-ro 217, Yuseong-gu, Daejeon 34113, Republic of Korea
40
Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
41
Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
42
Department of Physics, University of Chicago, 5720 South Ellis Avenue, Chicago, IL 60637, USA
43
Enrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
44
Cornell Center for Astrophysics and Planetary Science, Cornell University, Ithaca, NY 14853, USA
45
Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Nanjing 210008, People’s Republic of China
46
Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, SE-43992 Onsala,
Sweden
47
Dipartimento di Fisica “E. Pancini”, Universitá di Napoli “Federico II”, Compl. Univ. di Monte S. Angelo, Edificio G, Via Cinthia,
I-80126, Napoli, Italy
48
Institut für Theoretische Physik, Goethe-Universität Frankfurt, Max-von-Laue-Straße 1, D-60438 Frankfurt am Main, Germany
49
INFN Sez. di Napoli, Compl. Univ. di Monte S. Angelo, Edificio G, Via Cinthia, I-80126, Napoli, Italy
Monitoring the Morphology of M87
∗
in 2009–2017 with the EHT
3
50
Department of Physics, University of Pretoria, Lynnwood Road, Hatfield, Pretoria 0083, South Africa
51
Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, Grahamstown
6140, South Africa
52
LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195
Meudon, France
53
Department of History of Science, Harvard University, Cambridge, MA 02138, USA
54
Department of Physics, Harvard University, Cambridge, MA 02138, USA
55
Leiden Observatory—Allegro, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands
56
CP3-Origins, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark
57
Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro 1, Tonantzintla, Puebla, C.P. 72840, México
58
Consejo Nacional de Ciencia y Tecnología, Av. Insurgentes Sur 1582, 03940, Ciudad de México, México
59
Key Laboratory for Research in Galaxies and Cosmology, Chinese Academy of Sciences, Shanghai 200030, People’s Republic of China
60
NOVA Sub-mm Instrumentation Group, Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen,
The Netherlands
61
Department of Astronomy, School of Physics, Peking University, Beijing 100871, People’s Republic of China
62
Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People’s Republic of China
63
Instituto Nacional de Astrofísica, Óptica y Electrónica. Apartado Postal 51 y 216, 72000. Puebla Pue., México
64
ASTRAVEO LLC, PO Box 1668, MA 01931
65
Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstr. 1, D-85748 Garching, Germany
66
Institute for Astrophysical Research, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA
67
Astronomical Institute, St. Petersburg University, Universitetskij pr., 28, Petrodvorets,198504 St.Petersburg, Russia
68
Joint Institute for VLBI ERIC (JIVE), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
69
Kogakuin University of Technology & Engineering, Academic Support Center, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan
70
Physics Department, National Sun Yat-Sen University, No. 70, Lien-Hai Rd, Kaosiung City 80424, Taiwan, R.O.C
71
National Optical Astronomy Observatory, 950 North Cherry Ave., Tucson, AZ 85719, USA
72
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
73
School of Astronomy and Space Science, Nanjing University, Nanjing 210023, People’s Republic of China
74
Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Nanjing 210023, People’s Republic of China
75
Italian ALMA Regional Centre, INAF-Istituto di Radioastronomia, Via P. Gobetti 101, I-40129 Bologna, Italy
76
Department of Physics, National Taiwan University, No.1, Sect.4, Roosevelt Rd., Taipei 10617, Taiwan, R.O.C.
77
Yunnan Observatories, Chinese Academy of Sciences, 650011 Kunming, Yunnan Province, People’s Republic of China
78
Center for Astronomical Mega-Science, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing, 100012, People’s
Republic of China
79
Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, 650011 Kunming, People’s Republic
of China
80
School of Natural Sciences, Institute for Advanced Study, 1 Einstein Drive, Princeton, NJ 08540, USA
81
Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai, 200240, People’s Republic of China
82
Astronomy Department, Universidad de Concepción, Casilla 160-C, Concepción, Chile
83
Department of Astronomy, University of Massachusetts, 01003, Amherst, MA, USA
84
Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
85
Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
86
Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
87
Canadian Institute for Advanced Research, 180 Dundas St West, Toronto, ON M5G 1Z8, Canada
88
Department of Physics, National Taiwan Normal University, No. 88, Sec.4, Tingzhou Rd., Taipei 116, Taiwan, R.O.C.
89
School of Mathematics, Trinity College, Dublin 2, Ireland
90
Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, NJ 08544, USA
91
Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
92
Center for Theoretical Astrophysics, Los Alamos National Laboratory, Los Alamos, NM, 87545, USA
93
Instituto de Radioastronomía Milimétrica, IRAM, Avenida Divina Pastora 7, Local 20, E-18012, Granada, Spain
94
Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
95
Aalto University Department of Electronics and Nanoengineering, PL 15500, FI-00076 Aalto, Finland
96
Aalto University Metsähovi Radio Observatory, Metsähovintie 114, FI-02540 Kylmälä, Finland
97
Department of Astronomy, Yonsei University, Yonsei-ro 50, Seodaemun-gu, 03722 Seoul, Republic of Korea
98
Netherlands Organisation for Scientific Research (NWO), Postbus 93138, 2509 AC Den Haag, The Netherlands
99
Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980-8578, Japan
100
Astronomical Institute, Tohoku University, Sendai 980-8578, Japan
101
Department of Physics and Astronomy, Seoul National University, Gwanak-gu, Seoul 08826, Republic of Korea
4
102
Leiden Observatory, Leiden University, Postbus 2300, 9513 RA Leiden, The Netherlands
103
Physics Department, Brandeis University, 415 South Street, Waltham, MA 02453, USA
104
Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
105
School of Physics, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, People’s Republic of China
106
Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
107
School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049,
People’s Republic of China
108
Astronomy Department, University of Science and Technology of China, Hefei 230026, People’s Republic of China
Abstract
The Event Horizon Telescope (EHT) has recently delivered the first resolved images of M87
∗
, the
supermassive black hole in the center of the M87 galaxy. These images were produced using 230 GHz
observations performed in 2017 April. Additional observations are required to investigate the persis-
tence of the primary image feature – a ring with azimuthal brightness asymmetry – and to quantify
the image variability on event horizon scales. To address this need, we analyze M87
∗
data collected
with prototype EHT arrays in 2009, 2011, 2012, and 2013. While these observations do not contain
enough information to produce images, they are sufficient to constrain simple geometric models. We
develop a modeling approach based on the framework utilized for the 2017 EHT data analysis and
validate our procedures using synthetic data. Applying the same approach to the observational data
sets, we find the M87
∗
morphology in 2009–2017 to be consistent with a persistent asymmetric ring
of
∼
40
μ
as diameter. The position angle of the peak intensity varies in time. In particular, we find
a significant difference between the position angle measured in 2013 and 2017. These variations are
in broad agreement with predictions of a subset of general relativistic magnetohydrodynamic simula-
tions. We show that quantifying the variability across multiple observational epochs has the potential
to constrain the physical properties of the source, such as the accretion state or the black hole spin.
Keywords:
black holes – accretion, accretion disks – galaxies: active – galaxies: individual: M87 –
Galaxy: center – techniques: interferometric
1.
INTRODUCTION
The compact radio source in the center of the M87
galaxy, hereafter M87
∗
, has been observed at 1.3 mil-
limeter wavelength (230 GHz frequency) using very long
baseline interferometry (VLBI) since 2009. These obser-
vations, performed by early configurations of the Event
Horizon Telescope (EHT, Doeleman et al. 2009) ar-
ray, measured the size of the compact emission to be
∼
40
μ
as, with large systematic uncertainties related
to the limited baseline coverage (Doeleman et al. 2012;
Akiyama et al. 2015). The addition of new sites and
sensitivity improvements leading up to the April 2017
observations yielded the first resolved images of the
source (Event Horizon Telescope Collaboration et al.
2019a,b,c,d,e,f, hereafter EHTC I-VI). These images re-
vealed an asymmetric ring (a crescent) with a diameter
d
= 42
±
3
μ
as and a position angle of the bright side
φ
B
between
150
◦
and
200
◦
east of north (counterclockwise
from north/up as seen on the sky, EHTC VI), see the
left panel of Figure 1. The apparent size and appearance
of the observed ring agree with theoretical expectations
for a
6
.
5
×
10
9
M
black hole driving a magnetized ac-
cretion inflow/outflow system, inefficiently radiating via
synchrotron emission (Yuan & Narayan 2014, EHTC V).
Trajectories of the emitted photons are subject to strong
deflection in the vicinity of the event horizon, resulting
in a lensed ring-like feature seen by a distant observer –
the anticipated shadow of a black hole (Bardeen 1973;
Luminet 1979; Falcke et al. 2000; Broderick & Loeb
2009).
General
relativistic
magnetohydrodynamic
(GRMHD) simulations of relativistic plasma in the
accretion flow and jet-launching region close to the
black hole (EHTC V, Porth et al. 2019) predict that the
M87
∗
source structure will exhibit a prominent asym-
metric ring throughout multiple years of observations,
with a mean diameter
d
primarily determined by the
black hole mass-to-distance ratio and a position angle
φ
B
primarily determined by the orientation of the black
hole spin axis. The detailed appearance of M87
∗
may
also be influenced by many poorly constrained effects,
such as the black hole spin magnitude, magnetic field
structure in the accretion flow (Narayan et al. 2012,
EHTC V), the electron heating mechanism (e.g., Moś-
cibrodzka et al. 2016; Chael et al. 2018a), nonthermal
electrons (e.g., Davelaar et al. 2019), and misalignment
between the jet and the black hole spin (White et al.
2020; Chatterjee et al. 2020). Moreover, turbulence in
the accretion flow, perhaps driven by the magnetorota-
tional instability (Balbus & Hawley 1991), is expected
to cause stochastic variability in the image with correla-
tion timescales of up to a few weeks (
∼
dynamical time
for M87
∗
). The model uncertainties and expected time-
dependent variability of these theoretical predictions
Monitoring the Morphology of M87
∗
in 2009–2017 with the EHT
5
GRMHD
φ
jet
φ
spin
φ
B
,
exp
Observation
φ
jet
φ
B
,
exp
40
μ
as
Figure 1.
Left panel:
one of the images of M87
∗
obtained
in EHTC IV (see Section 4.2 for details). A 42
μ
as circle is
plotted with a dashed line for reference. The observed po-
sition angle of the approaching jet
φ
jet
is 288
◦
east of north
(Walker et al. 2018). Under the assumed physical interpre-
tation of the ring, we expect to find the bright side of the
crescent on average approximately 90
◦
clockwise from
φ
jet
(EHTC V). We assume a convention
φ
B
,
exp
= 198
◦
, indi-
cated with a blue dashed line.
Right panel:
a random snap-
shot (note that this is not a fit to the EHT image) from
a GRMHD simulation adopting the expected properties of
M87
∗
(Section 4.1). The spin vector of the black hole is
partially directed into the page, counteraligned with the ap-
proaching jet (and aligned with the deboosted receding jet);
its projection onto the observer’s screen is located at the
position angle of
φ
spin
=
φ
jet
−
180
◦
.
strongly motivate the need for additional observations
of M87
∗
, especially on timescales long enough to yield
uncorrelated snapshots of the turbulent flow.
To this end, we analyze archival EHT observations of
M87
∗
from observing campaigns in 2009, 2011, 2012,
and 2013. While these observations do not have enough
baseline coverage to form images (EHTC IV), they are
sufficient to constrain simple geometrical models, follow-
ing procedures similar to those presented in EHTC VI.
We employ asymmetric ring models that are motivated
by both results obtained with the mature 2017 array and
the expectation from GRMHD simulations that the ring
feature is persistent.
We begin, in Section 2, by summarizing the details of
these archival observations with the “proto-EHT” arrays.
In Section 3, we describe our procedure for fitting simple
geometrical models to these observations. In Section 4,
we test this procedure using synthetic proto-EHT obser-
vations of GRMHD snapshots and of the EHT images
of M87
∗
. We then use the same procedure to fit models
to the archival observations of M87
∗
in Section 5. We
discuss the implications of these results for our theo-
retical understanding of M87
∗
in Section 6, and briefly
summarize our findings in Section 7.
2.
OBSERVATIONS AND DATA
Our analysis covers five separate 1.3 mm VLBI observ-
ing campaigns conducted in 2009, 2011, 2012, 2013, and
2017. The M87
∗
data from 2011 and 2013 have not been
published previously. For all campaigns except 2012,
M87
∗
was observed on multiple nights. For the proto-
EHT data sets (2009–2013) we simultaneously utilize the
entire data set from each year, fitting to data from mul-
tiple days with a single source model, when available.
This is motivated by the M87
∗
dynamical timescale ar-
gument, little visibility amplitude variation reported by
EHTC III on a one-week timescale, as well as by the lim-
ited amount of available data and lack of evidence for
interday variability in the proto-EHT data sets. We use
incoherent averaging to estimate visibility amplitudes on
each scan (
∼
few minutes of continuous observation) and
bispectral averaging to estimate closure phases (Rogers
et al. 1995; Johnson et al. 2015; Fish et al. 2016). The
frequency setup in 2009–2013 consisted of two 480 MHz
bands, centered at 229.089 and 229.601 GHz. Whenever
both bands or both parallel-hand polarization compo-
nents were available, we incoherently averaged all simul-
taneous visibility amplitudes. The data sets are sum-
marized in Table 1, where the number of detections for
nonredundant baselines of different projected baseline
lengths is given, with the corresponding
(
u,v
)
-coverage
shown in Figure 2. Redundant baselines yield indepen-
dent observations of the same visibility. In Table 1 we
also indicate the number of available nonredundant clo-
sure phases (CPs, not counting redundant and intrasite
baselines, minimal set, see Blackburn et al. 2020). As
is the case for non-phase-referenced VLBI observations
(Thompson et al. 2017), we do not have access to abso-
lute visibility phases. All visibility amplitudes observed
in 2009–2013 are presented in Figure 3.
A more detailed summary of the observational setup
of the proto-EHT array in 2009–2013 and the associated
data reduction procedures can be found in Fish et al.
(2016). All data sets discussed in this paper are publicly
available
1
.
2.1.
2009–2012
Prior to 2013, the proto-EHT array included tele-
scopes at three geographical locations: (1) the Com-
bined Array for Research in Millimeter-wave Astronomy
(CARMA, CA) in Cedar Flat, California, (2) the Sub-
millimeter Telescope (SMT, AZ) on Mt. Graham in Ari-
zona, and (3) the Submillimeter Array (SMA, SM), the
James Clerk Maxwell Telescope (JCMT, JC), and the
Caltech Submillimeter Observatory (CSO, CS) on Mau-
nakea in Hawai’i. These arrays were strongly east-west
oriented, and the longest projected baselines, between
SMT and Hawai’i, reached about 3.5 G
λ
, corresponding
to the instrument resolution (maximum fringe spacing)
of
∼
60
μ
as.
1
https://eventhorizontelescope.org/for-astronomers/data
6
Table 1.
M87
∗
data sets analyzed in this paper.
Detections on Nonredundant Baselines
Year
Telescopes
Dates
Baselines
a
Zero Short Medium
b
Long
c
Total CPs
<
0
.
1
G
λ <
1
G
λ <
3
.
6
G
λ >
3
.
6
G
λ >
0
.
1
G
λ
2009
CA, AZ, JC
Apr 5, 6
3/3/3
—
12
16/5
—
28
—
2011
CA, AZ, JC, SM, CS
Mar 29, 31; Apr 1, 2, 4 10/6/3
52
33
21/6
—
54
—
2012
CA, AZ, SM
Mar 21
3/3/3
14
11
19/6
—
44
7
2013
CA, AZ, SM, JC, AP
Mar 21 – 23, Mar 26 10/7/5
39
41
23/4
19/1
83
—
2017 AZ, SM, JC, AP, LM, PV, AA
Apr 6
d
21/21/10
24
—
33/13 92/16
125
67
2017 AZ, SM, JC, AP, LM, PV, AA
Apr 11
d
21/21/10
22
—
28/9
72/16
100 54
a
theoretically available / with detections / nonredundant, nonzero with detections,
b
all / SMT-Hawai’i,
c
all / SMT-Chile,
d
single-day data set
The 2011 observations of M87
∗
have not been pub-
lished but follow the data reduction procedures de-
scribed in Lu et al. (2013). The 2009 and 2012 observa-
tions and data processing of M87
∗
have been published
in Doeleman et al. (2012) and Akiyama et al. (2015),
respectively. However, our analysis uses a modified pro-
cessing of the 2012 data because the original processing
erroneously applied the same correction for atmospheric
opacity at the SMT twice.
2
The SMT calibration pro-
cedures have been updated since then (Issaoun et al.
2017).
Each observation included multiple subarrays of
CARMA as well as simultaneous measurements of
the total source flux density with CARMA acting as
a connected-element interferometer; these properties
then allow the CARMA amplitude gains to be “net-
work calibrated” (Fish et al. 2011; Johnson et al. 2015,
EHTC III). Of these three observing campaigns, only
2012 provides closure phase information for M87
∗
, and
all closure phases measured on the single, narrow tri-
angle SMT–SMA–CARMA were consistent with zero to
within
2
σ
(Akiyama et al. 2015), see Figure 4.
2.2.
2013
The 2013 observing epoch did not include the CSO,
but added the Atacama Pathfinder Experiment fa-
cility (APEX, AP) in the Atacama Desert in Chile.
This additional site brought for the first time the
long (
≈
5
−
6 G
λ
) baselines CARMA–APEX and SMT–
APEX, that are roughly orthogonal to the CARMA–
Hawai’i and SMT–Hawai’i baselines, see Figure 2. The
addition of APEX increased the instrument resolution
(maximum fringe spacing) to
∼
35
μ
as. While the 2013
2
An opacity correction raises visibility amplitudes on SMT base-
lines by
∼
10% in nominal conditions; our visibility amplitudes
on SMT baselines are, thus, slightly lower than those reported
by Akiyama et al. (2015). However, the calibration error does
not change the primary conclusions of Akiyama et al. (2015).
observations of Sgr A
∗
were presented in several publi-
cations (Johnson et al. 2015; Fish et al. 2016; Lu et al.
2018), the M87
∗
observations obtained during the 2013
campaign have not been published previously.
The proto-EHT array observed M87
∗
on March 21st,
22nd, 23rd, and 26th 2013. CARMA–APEX detections
were found on March 22nd (11 detections) and 23rd
(7 detections) with a single SMT–APEX detection on
March 23rd. March 23rd (MJD 36374) was the only
day with detections on baselines to each of the four
geographical sites. No detections between Hawai’i and
APEX were found, and there were no simultaneous de-
tections over a closed triangle that would allow for the
measurement of closure phase.
2.3.
2017
In 2017, the EHT observed M87
∗
with five geo-
graphical sites (EHTC I; EHTC II), without CSO and
CARMA, but with the addition of the Large Millime-
ter Telescope Alfonso Serrano (LMT, LM) on the Vol-
cán Sierra Negra in Mexico, the IRAM 30-m telescope
(PV) on Pico Veleta in Spain, and the phased-up At-
acama Large Millimeter/submillimeter Array (ALMA,
AA, Matthews et al. 2018; Goddi et al. 2019). The ex-
pansion of the array resulted in significant improvements
in
(
u,v
)
-coverage, shown with gray lines in Figure 2, and
instrument resolution raised to
∼
25
μ
as. In addition to
hardware setup developments (EHTC II), the recorded
bandwidth was increased from 2
×
0.5 GHz to 2
×
2 GHz
(226-230 GHz). The 2017 data processing pipeline used
ALMA as an anchor station (EHTC III). Its high sensi-
tivity greatly improved the signal phase stability (Black-
burn et al. 2019; Janssen et al. 2019, EHTC III) and en-
abled data analysis based on robustly detected closure
quantities obtained from coherently averaged visibilities
(EHTC IV, Blackburn et al. 2020) rather than on vis-
ibility amplitudes alone. These improvements allowed
for an unambiguous analysis of the M87
∗
image by con-