First M87 Event Horizon Telescope Results. IX. Detection of Near-horizon Circular Polarization
Authors:   The 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 , Chang Dominic O. , 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 , Dahale Rohan , Davelaar Jordy , De Laurentis Mariafelicia , 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 , Foschi Marianna , 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 Joana A. , Kramer Michael , Krichbaum Thomas P. , Kuo Cheng-Yu , La Bella Noemi , 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. , Lowitz Amy E. , Lu 路 Ru-Sen 如森 , MacDonald Nicholas R. , 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 , Mulaudzi Wanga , Müller Cornelia , Müller Hendrik , 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 , Özel Feryal , 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. , Psaltis Dimitrios , Pu Hung-Yi , Ramakrishnan Venkatessh , Rao Ramprasad , Rawlings Mark G. , Raymond Alexander W. , Rezzolla Luciano , Ricarte Angelo , Ripperda Bart , Roelofs Freek , Rogers Alan , Romero-Cañizales Cristina , Ros Eduardo , 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 , Sosapanta Salas León David , Souccar Kamal , Sun 孙 He 赫 , Tazaki Fumie , Tetarenko Alexandra J. , Tiede Paul , Tilanus Remo P. J. , Titus Michael , Torne Pablo , Toscano Teresa , 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 , Washington Jasmin E. , Weintroub Jonathan , Wharton Robert , Wielgus Maciek , Wiik Kaj , Witzel Gunther , Wondrak Michael F. , Wong George N. , Wu 吴 Qingwen 庆文 , Yadlapalli Nitika , Yamaguchi Paul , Yfantis Aristomenis , Yoon Doosoo , Young André , Young Ken , Younsi Ziri , Yu 于 Wei 威 , Yuan 袁 Feng 峰 , Yuan 袁 Ye-Fei 业飞 , Zensus J. Anton , Zhang Shuo , Zhao Guang-Yao , Zhao 赵 Shan-Shan 杉杉 
Published: November 2023

Event Horizon Telescope (EHT) observations have revealed a bright ring of emission around the supermassive black hole at the center of the M87 galaxy. EHT images in linear polarization have further identified a coherent spiral pattern around the black hole, produced from ordered magnetic fields threading the emitting plasma. Here we present the first analysis of circular polarization using EHT data, acquired in 2017, which can potentially provide additional insights into the magnetic fields and plasma composition near the black hole. Interferometric closure quantities provide convincing evidence for the presence of circularly polarized emission on event-horizon scales. We produce images of the circular polarization using both traditional and newly developed methods. All methods find a moderate level of resolved circular polarization across the image (〈∣v∣〉 < 3.7%), consistent with the low image-integrated circular polarization fraction measured by the Atacama Large Millimeter/submillimeter Array (∣v
int∣ < 1%). Despite this broad agreement, the methods show substantial variation in the morphology of the circularly polarized emission, indicating that our conclusions are strongly dependent on the imaging assumptions because of the limited baseline coverage, uncertain telescope gain calibration, and weakly polarized signal. We include this upper limit in an updated comparison to general relativistic magnetohydrodynamic simulation models. This analysis reinforces the previously reported preference for magnetically arrested accretion flow models. We find that most simulations naturally produce a low level of circular polarization consistent with our upper limit and that Faraday conversion is likely the dominant production mechanism for circular polarization at 230 GHz in M87*.