First Sagittarius A* Event Horizon Telescope Results. IV. Variability, Morphology, and Black Hole Mass
Authors: Event Horizon Telescope Collaboration  , Akiyama Kazunori , Alberdi Antxon , Alef Walter , Algaba Juan Carlos , Anantua Richard , Asada Keiichi , Azulay Rebecca , Bach Uwe , Baczko Anne-Kathrin , Ball David , Baloković Mislav , Barrett John , Bauböck Michi , Benson Bradford A. , Bintley Dan , Blackburn Lindy , Blundell Raymond , Bouman Katherine L. , Bower Geoffrey C. , Boyce Hope , Bremer Michael , Brinkerink Christiaan D. , Brissenden Roger , Britzen Silke , Broderick Avery E. , Broguiere Dominique , Bronzwaer Thomas , Bustamante Sandra , Byun Do-Young , Carlstrom John E. , Ceccobello Chiara , Chael Andrew , Chan Chi-kwan , Chatterjee Koushik , Chatterjee Shami , Chen Ming-Tang , Chen 陈 Yongjun 永军 , Cheng Xiaopeng , Cho Ilje , Christian Pierre , Conroy Nicholas S. , Conway John E. , Cordes James M. , Crawford Thomas M. , Crew Geoffrey B. , Cruz-Osorio Alejandro , Cui 崔 Yuzhu 玉竹 , Davelaar Jordy , Laurentis Mariafelicia De , Deane Roger , Dempsey Jessica , Desvignes Gregory , Dexter Jason , Dhruv Vedant , Doeleman Sheperd S. , Dougal Sean , Dzib Sergio A. , Eatough Ralph P. , Emami Razieh , Falcke Heino , Farah Joseph , Fish Vincent L. , Fomalont Ed , Ford H. Alyson , Fraga-Encinas Raquel , Freeman William T. , Friberg Per , Fromm Christian M. , Fuentes Antonio , Galison Peter , Gammie Charles F. , García Roberto , Gentaz Olivier , Georgiev Boris , Goddi Ciriaco , Gold Roman , Gómez-Ruiz Arturo I. , Gómez José L. , Gu 顾 Minfeng 敏峰 , Gurwell Mark , Hada Kazuhiro , Haggard Daryl , Haworth Kari , Hecht Michael H. , Hesper Ronald , Heumann Dirk , Ho 何 Luis C. 子山 , Ho Paul , Honma Mareki , Huang Chih-Wei L. , Huang 黄 Lei 磊 , Hughes David H. , Ikeda Shiro , Impellizzeri C. M. Violette , Inoue Makoto , Issaoun Sara , James David J. , Jannuzi Buell T. , Janssen Michael , Jeter Britton , Jiang 江 Wu 悟 , Jiménez-Rosales Alejandra , Johnson Michael D. , Jorstad Svetlana , Joshi Abhishek V. , Jung Taehyun , Karami Mansour , Karuppusamy Ramesh , Kawashima Tomohisa , Keating Garrett K. , Kettenis Mark , Kim Dong-Jin , Kim Jae-Young , Kim Jongsoo , Kim Junhan , Kino Motoki , Koay Jun Yi , Kocherlakota Prashant , Kofuji Yutaro , Koch Patrick M. , Koyama Shoko , Kramer Carsten , Kramer Michael , Krichbaum Thomas P. , Kuo Cheng-Yu , Bella Noemi La , Lauer Tod R. , Lee Daeyoung , Lee Sang-Sung , Leung Po Kin , Levis Aviad , Li 李 Zhiyuan 志远 , Lico Rocco , Lindahl Greg , Lindqvist Michael , Lisakov Mikhail , Liu 刘 Jun 俊 , Liu Kuo , Liuzzo Elisabetta , Lo Wen-Ping , Lobanov Andrei P. , Loinard Laurent , Lonsdale Colin J. , Lu 路 Ru-Sen 如森 , Mao 毛 Jirong 基荣 , Marchili Nicola , Markoff Sera , Marrone Daniel P. , Marscher Alan P. , Martí-Vidal Iván , Matsushita Satoki , Matthews Lynn D. , Medeiros Lia , Menten Karl M. , Michalik Daniel , Mizuno Izumi , Mizuno Yosuke , Moran James M. , Moriyama Kotaro , Moscibrodzka Monika , Müller Cornelia , Mus Alejandro , Musoke Gibwa , Myserlis Ioannis , Nadolski Andrew , Nagai Hiroshi , Nagar Neil M. , Nakamura Masanori , Narayan Ramesh , Narayanan Gopal , Natarajan Iniyan , Nathanail Antonios , Fuentes Santiago Navarro , Neilsen Joey , Neri Roberto , Ni Chunchong , Noutsos Aristeidis , Nowak Michael A. , Oh Junghwan , Okino Hiroki , Olivares Héctor , Ortiz-León Gisela N. , Oyama Tomoaki , Palumbo Daniel C. M. , Paraschos Georgios Filippos , Park Jongho , Parsons Harriet , Patel Nimesh , Pen Ue-Li , Pesce Dominic W. , Piétu Vincent , Plambeck Richard , PopStefanija Aleksandar , Porth Oliver , Pötzl Felix M. , Prather Ben , Preciado-López Jorge A. , Pu Hung-Yi , Ramakrishnan Venkatessh , Rao Ramprasad , Rawlings Mark G. , Raymond Alexander W. , Rezzolla Luciano , Ricarte Angelo , Ripperda Bart , Roelofs Freek , Rogers Alan , Ros Eduardo , Romero-Cañizales Cristina , Roshanineshat Arash , Rottmann Helge , Roy Alan L. , Ruiz Ignacio , Ruszczyk Chet , Rygl Kazi L. J. , Sánchez Salvador , Sánchez-Argüelles David , Sánchez-Portal Miguel , Sasada Mahito , Satapathy Kaushik , Savolainen Tuomas , Schloerb F. Peter , Schonfeld Jonathan , Schuster Karl-Friedrich , Shao Lijing , Shen 沈 Zhiqiang 志强 , Small Des , Sohn Bong Won , SooHoo Jason , Souccar Kamal , Sun 孙 He 赫 , Tazaki Fumie , Tetarenko Alexandra J. , Tiede Paul , Tilanus Remo P. J. , Titus Michael , Torne Pablo , Traianou Efthalia , Trent Tyler , Trippe Sascha , Turk Matthew , van Bemmel Ilse , van Langevelde Huib Jan , van Rossum Daniel R. , Vos Jesse , Wagner Jan , Ward-Thompson Derek , Wardle John , Weintroub Jonathan , Wex Norbert , Wharton Robert , Wielgus Maciek , Wiik Kaj , Witzel Gunther , Wondrak Michael F. , Wong George N. , Wu 吴 Qingwen 庆文 , Yamaguchi Paul , Yoon Doosoo , Young André , Young Ken , Younsi Ziri , Yuan 袁 Feng 峰 , Yuan 袁 Ye-Fei 业飞 , Zensus J. Anton , Zhang Shuo , Zhao Guang-Yao , Zhao 赵 Shan-Shan 杉杉 , Chang Dominic O. 
Published: May 2022

Abstract
In this paper we quantify the temporal variability and image morphology of the horizon-scale emission from Sgr A*, as observed by the EHT in 2017 April at a wavelength of 1.3 mm. We find that the Sgr A* data exhibit variability that exceeds what can be explained by the uncertainties in the data or by the effects of interstellar scattering. The magnitude of this variability can be a substantial fraction of the correlated flux density, reaching ∼100% on some baselines. Through an exploration of simple geometric source models, we demonstrate that ring-like morphologies provide better fits to the Sgr A* data than do other morphologies with comparable complexity. We develop two strategies for fitting static geometric ring models to the time-variable Sgr A* data; one strategy fits models to short segments of data over which the source is static and averages these independent fits, while the other fits models to the full data set using a parametric model for the structural variability power spectrum around the average source structure. Both geometric modeling and image-domain feature extraction techniques determine the ring diameter to be 51.8 ± 2.3 μas (68% credible intervals), with the ring thickness constrained to have an FWHM between ∼30% and 50% of the ring diameter. To bring the diameter measurements to a common physical scale, we calibrate them using synthetic data generated from GRMHD simulations. This calibration constrains the angular size of the gravitational radius to be

4.8

−
0.7

+
1.4

μas, which we combine with an independent distance measurement from maser parallaxes to determine the mass of Sgr A* to be

4.0

−
0.6

+
1.1

×

10

6

M
⊙.