First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole
Authors:   The Event Horizon Telescope Collaboration, Akiyama Kazunori , Alberdi Antxon , Alef Walter , Asada Keiichi , Azulay Rebecca , Baczko Anne-Kathrin , Ball David , Baloković Mislav , Barrett John , Bintley Dan , Blackburn Lindy , Boland Wilfred , Bouman Katherine L. , Bower Geoffrey C. , Bremer Michael , Brinkerink Christiaan D. , Brissenden Roger , Britzen Silke , Broderick Avery E. , Broguiere Dominique , Bronzwaer Thomas , Byun Do-Young , Carlstrom John E. , Chael Andrew , Chan Chi-kwan , Chatterjee Shami , Chatterjee Koushik , Chen Ming-Tang , Chen 陈 Yongjun 永军 , Cho Ilje , Christian Pierre , Conway John E. , Cordes James M. , Crew Geoffrey B. , Cui Yuzhu , Davelaar Jordy , De Laurentis Mariafelicia , Deane Roger , Dempsey Jessica , Desvignes Gregory , Dexter Jason , Doeleman Sheperd S. , Eatough Ralph P. , Falcke Heino , Fish Vincent L. , Fomalont Ed , Fraga-Encinas Raquel , Freeman William T. , Friberg Per , Fromm Christian M. , Gómez José L. , Galison Peter , Gammie Charles F. , García Roberto , Gentaz Olivier , Georgiev Boris , Goddi Ciriaco , Gold Roman , Gu 顾 Minfeng 敏峰 , Gurwell Mark , Hada Kazuhiro , Hecht Michael H. , Hesper Ronald , Ho 何 Luis C. 子山 , Ho Paul , Honma Mareki , Huang Chih-Wei L. , Huang 黄 Lei 磊 , H. Hughes David , Ikeda Shiro , Inoue Makoto , Issaoun Sara , James David J. , Jannuzi Buell T. , Janssen Michael , Jeter Britton , Jiang 江 Wu 悟 , Johnson Michael D. , Jorstad Svetlana , Jung Taehyun , Karami Mansour , Karuppusamy Ramesh , Kawashima Tomohisa , Keating Garrett K. , Kettenis Mark , Kim Jae-Young , Kim Junhan , Kim Jongsoo , Kino Motoki , Koay Jun Yi , Koch Patrick M. , Koyama Shoko , Kramer Michael , Kramer Carsten , Krichbaum Thomas P. , Kuo Cheng-Yu , Lauer Tod R. , Lee Sang-Sung , Li 李 Yan-Rong 彦荣 , Li 李 Zhiyuan 志远 , Lindqvist Michael , Liu Kuo , Liuzzo Elisabetta , Lo Wen-Ping , Lobanov Andrei P. , Loinard Laurent , Lonsdale Colin , Lu 路 Ru-Sen 如森 , MacDonald Nicholas R. , Mao 毛 Jirong 基荣 , Markoff Sera , Marrone Daniel P. , Marscher Alan P. , Martí-Vidal Iván , Matsushita Satoki , Matthews Lynn D. , Medeiros Lia , Menten Karl M. , Mizuno Yosuke , Mizuno Izumi , Moran James M. , Moriyama Kotaro , Moscibrodzka Monika , Müller Cornelia , Nagai Hiroshi , Nagar Neil M. , Nakamura Masanori , Narayan Ramesh , Narayanan Gopal , Natarajan Iniyan , Neri Roberto , Ni Chunchong , Noutsos Aristeidis , Okino Hiroki , Olivares Héctor , Oyama Tomoaki , Özel Feryal , Palumbo Daniel C. M. , Patel Nimesh , Pen Ue-Li , Pesce Dominic W. , Piétu Vincent , Plambeck Richard , PopStefanija Aleksandar , Porth Oliver , Prather Ben , Preciado-López Jorge A. , Psaltis Dimitrios , Pu Hung-Yi , Ramakrishnan Venkatessh , Rao Ramprasad , Rawlings Mark G. , Raymond Alexander W. , Rezzolla Luciano , Ripperda Bart , Roelofs Freek , Rogers Alan , Ros Eduardo , Rose Mel , Roshanineshat Arash , Rottmann Helge , Roy Alan L. , Ruszczyk Chet , Ryan Benjamin R. , Rygl Kazi L. J. , Sánchez Salvador , Sánchez-Arguelles David , Sasada Mahito , Savolainen Tuomas , Schloerb F. Peter , Schuster Karl-Friedrich , Shao Lijing , Shen 沈 Zhiqiang 志强 , Small Des , Sohn Bong Won , SooHoo Jason , Tazaki Fumie , Tiede Paul , Tilanus Remo P. J. , Titus Michael , Toma Kenji , Torne Pablo , Trent Tyler , Trippe Sascha , Tsuda Shuichiro , van Bemmel Ilse , van Langevelde Huib Jan , van Rossum Daniel R. , Wagner Jan , Wardle John , Weintroub Jonathan , Wex Norbert , Wharton Robert , Wielgus Maciek , Wong George N. , Wu 吴 Qingwen 庆文 , Young André , Young Ken , Younsi Ziri , Yuan 袁 Feng 峰 , Yuan 袁 Ye-Fei 业飞 , Zensus J. Anton , Zhao Guangyao , Zhao Shan-Shan , Zhu Ziyan , Farah Joseph R. , Meyer-Zhao Zheng , Michalik Daniel , Nadolski Andrew , Nishioka Hiroaki , Pradel Nicolas , Primiani Rurik A. , Souccar Kamal , Vertatschitsch Laura , Yamaguchi Paul 
Published: April 2019

Abstract
We present the first Event Horizon Telescope (EHT) images of M87, using observations from April 2017 at 1.3 mm wavelength. These images show a prominent ring with a diameter of ∼40 μas, consistent with the size and shape of the lensed photon orbit encircling the “shadow” of a supermassive black hole. The ring is persistent across four observing nights and shows enhanced brightness in the south. To assess the reliability of these results, we implemented a two-stage imaging procedure. In the first stage, four teams, each blind to the others’ work, produced images of M87 using both an established method (CLEAN) and a newer technique (regularized maximum likelihood). This stage allowed us to avoid shared human bias and to assess common features among independent reconstructions. In the second stage, we reconstructed synthetic data from a large survey of imaging parameters and then compared the results with the corresponding ground truth images. This stage allowed us to select parameters objectively to use when reconstructing images of M87. Across all tests in both stages, the ring diameter and asymmetry remained stable, insensitive to the choice of imaging technique. We describe the EHT imaging procedures, the primary image features in M87, and the dependence of these features on imaging assumptions.