Polarimetric Properties of Event Horizon Telescope Targets from ALMA
Authors: Goddi Ciriaco , Martí-Vidal Iván , Messias Hugo , Bower Geoffrey C. , Broderick Avery E. , Dexter Jason , Marrone Daniel P. , Moscibrodzka Monika , Nagai Hiroshi , Algaba Juan Carlos , Asada Keiichi , Crew Geoffrey B. , Gómez José L. , Impellizzeri C. M. Violette , Janssen Michael , Kadler Matthias , Krichbaum Thomas P. , Lico Rocco , Matthews Lynn D. , Nathanail Antonios , Ricarte Angelo , Ros Eduardo , Younsi Ziri , Akiyama Kazunori , Alberdi Antxon , Alef Walter , Anantua Richard , Azulay Rebecca , Baczko Anne-Kathrin , Ball David , Baloković Mislav , Barrett John , Benson Bradford A. , Bintley Dan , Blackburn Lindy , Blundell Raymond , Boland Wilfred , Bouman Katherine L. , Boyce Hope , Bremer Michael , Brinkerink Christiaan D. , Brissenden Roger , Britzen Silke , Broguiere Dominique , Bronzwaer Thomas , Byun Do-Young , Carlstrom John E. , Chael Andrew , Chan Chi-kwan , Chatterjee Shami , Chatterjee Koushik , Chen Ming-Tang , Chen 陈永 Yongjun 军 , Chesler Paul M. , Cho Ilje , Christian Pierre , Conway John E. , Cordes James M. , Crawford Thomas M. , Cruz-Osorio Alejandro , Cui Yuzhu , Davelaar Jordy , De Laurentis Mariafelicia , Deane Roger , Dempsey Jessica , Desvignes Gregory , Doeleman Sheperd S. , Eatough Ralph P. , 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 , Gold Roman , Gómez-Ruiz Arturo I. , Gu 顾敏 Minfeng 峰 , Gurwell Mark , Hada Kazuhiro , Haggard Daryl , Hecht Michael H. , Hesper Ronald , Ho 何子 Luis C. 山 , Ho Paul , Honma Mareki , Huang Chih-Wei L. , Huang 黄 Lei 磊 , Hughes David H. , Inoue Makoto , Issaoun Sara , James David J. , Jannuzi Buell T. , Jeter Britton , Jiang 江 Wu 悟 , Jimenez-Rosales Alejandra , Johnson Michael D. , Jorstad Svetlana , 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 , Kofuji Yutaro , Koch Patrick M. , Koyama Shoko , Kramer Michael , Kramer Carsten , Kuo Cheng-Yu , Lauer Tod R. , Lee Sang-Sung , Levis Aviad , Li 李彦 Yan-Rong 荣 , Li 李志 Zhiyuan 远 , Lindqvist Michael , Lindahl Greg , Liu 刘 Jun 俊 , Liu Kuo , Liuzzo Elisabetta , Lo Wen-Ping , Lobanov Andrei P. , Loinard Laurent , Lonsdale Colin , Lu 路如 Ru-Sen 森 , MacDonald Nicholas R. , Mao 毛基 Jirong 荣 , Marchili Nicola , Markoff Sera , Marscher Alan P. , Matsushita Satoki , Medeiros Lia , Menten Karl M. , Mizuno Izumi , Mizuno Yosuke , Moran James M. , Moriyama Kotaro , Müller Cornelia , Musoke Gibwa , Mejías Alejandro Mus , Nagar Neil M. , Nakamura Masanori , Narayan Ramesh , Narayanan Gopal , Natarajan Iniyan , Neilsen Joey , Neri Roberto , Ni Chunchong , Noutsos Aristeidis , Nowak Michael A. , Okino Hiroki , Olivares Héctor , Ortiz-León Gisela N. , Oyama Tomoaki , Özel Feryal , Palumbo Daniel C. M. , Park Jongho , 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 , Ripperda Bart , Roelofs Freek , Rogers Alan , Rose Mel , Roshanineshat Arash , Rottmann Helge , Roy Alan L. , Ruszczyk Chet , Rygl Kazi L. J. , Sánchez Salvador , Sánchez-Arguelles David , Sasada Mahito , Savolainen Tuomas , Schloerb F. Peter , Schuster Karl-Friedrich , Shao Lijing , Shen 沈志 Zhiqiang 强 , Small Des , Sohn Bong Won , SooHoo Jason , Sun 孙 He 赫 , Tazaki Fumie , Tetarenko Alexandra J. , Tiede Paul , Tilanus Remo P. J. , Titus Michael , Toma Kenji , Torne Pablo , Trent Tyler , Traianou Efthalia , Trippe Sascha , Bemmel Ilse van , van Langevelde Huib Jan , van Rossum Daniel R. , Wagner Jan , Ward-Thompson Derek , Wardle John , Weintroub Jonathan , Wex Norbert , Wharton Robert , Wielgus Maciek , Wong George N. , Wu 吴庆 Qingwen 文 , Yoon Doosoo , Young André , Young Ken , Yuan 袁 Feng 峰 , Yuan 袁业 Ye-Fei 飞 , Zensus J. Anton , Zhao Guang-Yao , Zhao Shan-Shan , Bruni Gabriele , Gopakumar A. , Hernández-Gómez Antonio , Herrero-Illana Ruben , Ingram Adam , Komossa S. , Kovalev Y. Y. , Muders Dirk , Perucho Manel , Rösch Florian , Valtonen Mauri 
Published: March 2021

Abstract
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 2017 April in the λ3 mm and λ1.3 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 active galactic nuclei (AGNs), in the two bands at several epochs in a time window of 10 days. We detect high linear polarization fractions (2%–15%) and large rotation measures (RM > 103.3–105.5 rad m−2), confirming the trends of previous AGN studies at millimeter wavelengths. We find that blazars are more strongly polarized than other AGNs in the sample, while exhibiting (on average) 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 ± 0.3) × 105 rad m−2 at 1.3 mm, consistent with measurements over the past decade and, for the first time, an RM of (–2.1 ± 0.1) × 105 rad m−2 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 millimeter wavelengths, which undergoes significant changes in magnitude and sign reversals on a one year timescale, spanning the range from −1.2 to 0.3 × 105 rad m−2 at 3 mm and −4.1 to 1.5 × 105 rad m−2 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.