Millimeter Light Curves of Sagittarius A* Observed during the 2017 Event Horizon Telescope Campaign
Authors: Wielgus Maciek , Marchili Nicola , Martí-Vidal Iván , Keating Garrett K. , Ramakrishnan Venkatessh , Tiede Paul , Fomalont Ed , Issaoun Sara , Neilsen Joey , Nowak Michael A. , Blackburn Lindy , Gammie Charles F. , Goddi Ciriaco , Haggard Daryl , Lee Daeyoung , Moscibrodzka Monika , Tetarenko Alexandra J. , Bower Geoffrey C. , Chan Chi-kwan , Chatterjee Koushik , Chesler Paul M. , Dexter Jason , Doeleman Sheperd S. , Georgiev Boris , Gurwell Mark , Johnson Michael D. , Marrone Daniel P. , Mus Alejandro , Psaltis Dimitrios , Ripperda Bart , Witzel Gunther , Akiyama Kazunori , Alberdi Antxon , Alef Walter , Carlos Algaba Juan , Anantua Richard , Asada Keiichi , Azulay Rebecca , Bach Uwe , Baczko Anne-Kathrin , Ball David , Baloković Mislav , Barrett John , Bauböck Michi , Benson Bradford A. , Bintley Dan , Blundell Raymond , Boland Wilfred , Bouman Katherine L. , 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 , Chatterjee Shami , Chen Ming-Tang , Chen 陈 Yongjun 永军 , Cho Ilje , Christian Pierre , Conroy Nicholas S. , Conway John E. , Cordes James M. , Crawford Thomas M. , Crew Geoffrey B. , Cruz-Osorio Alejandro , Cui Yuzhu , Davelaar Jordy , De Laurentis Mariafelicia , Deane Roger , Dempsey Jessica , Desvignes Gregory , Dhruv Vedant , Dzib Sergio A. , Eatough Ralph P. , Emami Razieh , Falcke Heino , Farah Joseph , Fish Vincent L. , Alyson Ford H. , Fraga-Encinas Raquel , Freeman William T. , Friberg Per , Fromm Christian M. , Fuentes Antonio , Galison Peter , García Roberto , Gentaz Olivier , Gold Roman , Gómez-Ruiz Arturo I. , Gómez José L. , Gu 顾 Minfeng 敏峰 , Hada Kazuhiro , Haworth Kari , Hecht Michael H. , Hesper Ronald , Ho 何 Luis C. 子山 , Ho Paul , Honma Mareki , Huang Chih-Wei L. , Huang 黄 Lei 磊 , Hughes David H. , Ikeda Shiro , Violette Impellizzeri C. M. , Inoue Makoto , James David J. , Jannuzi Buell T. , Janssen Michael , Jeter Britton , Jiang 江 Wu 悟 , Jiménez-Rosales Alejandra , Jorstad Svetlana , Joshi Abhishek V. , Jung Taehyun , Karami Mansour , Karuppusamy Ramesh , Kawashima Tomohisa , Kettenis Mark , Kim Dong-Jin , Kim Jae-Young , Kim Jongsoo , Kim Junhan , Kino Motoki , Yi Koay Jun , Kocherlakota Prashant , Kofuji Yutaro , Koch Patrick M. , Koyama Shoko , Kramer Carsten , Kramer Michael , Krichbaum Thomas P. , Kuo Cheng-Yu , La Bella Noemi , Lauer Tod R. , Lee Sang-Sung , Kin Leung Po , 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 , Lu 路 Ru-Sen 如森 , Mao 毛 Jirong 基荣 , Markoff Sera , Marscher Alan P. , Matsushita Satoki , Matthews Lynn D. , Medeiros Lia , Menten Karl M. , Michalik Daniel , Mizuno Izumi , Mizuno Yosuke , Moran James M. , Moriyama Kotaro , Müller Cornelia , Musoke Gibwa , Myserlis Ioannis , Nadolski Andrew , Nagai Hiroshi , Nagar Neil M. , Nakamura Masanori , Narayan Ramesh , Narayanan Gopal , Natarajan Iniyan , Nathanail Antonios , Navarro Fuentes Santiago , Neri Roberto , Ni Chunchong , Noutsos Aristeidis , Oh Junghwan , Okino Hiroki , Olivares Héctor , Ortiz-León Gisela N. , Oyama Tomoaki , Özel Feryal , Palumbo Daniel C. M. , Filippos Paraschos Georgios , 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 , Rao Ramprasad , Rawlings Mark G. , Raymond Alexander W. , Rezzolla Luciano , Ricarte Angelo , Roelofs Freek , Rogers Alan , Ros Eduardo , Romero-Canizales 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 , Peter Schloerb F. , Schuster Karl-Friedrich , Shao Lijing , Shen 沈 Zhiqiang 志强 , Small Des , Won Sohn Bong , SooHoo Jason , Souccar Kamal , Sun 孙 He 赫 , Tazaki Fumie , Tilanus Remo P. J. , Titus Michael , Torne Pablo , Traianou Efthalia , Trent Tyler , Trippe Sascha , van Bemmel Ilse , Jan van Langevelde Huib , van Rossum Daniel R. , Vos Jesse , Wagner Jan , Ward-Thompson Derek , Wardle John , Weintroub Jonathan , Wex Norbert , Wharton Robert , Wiik Kaj , 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 , Anton Zensus J. , Zhang Shuo , Zhao Guang-Yao , Zhao Shan-Shan 
Published: May 2022

Abstract
The Event Horizon Telescope (EHT) observed the compact radio source, Sagittarius A* (Sgr A*), in the Galactic Center on 2017 April 5–11 in the 1.3 mm wavelength band. At the same time, interferometric array data from the Atacama Large Millimeter/submillimeter Array and the Submillimeter Array were collected, providing Sgr A* light curves simultaneous with the EHT observations. These data sets, complementing the EHT very long baseline interferometry, are characterized by a cadence and signal-to-noise ratio previously unattainable for Sgr A* at millimeter wavelengths, and they allow for the investigation of source variability on timescales as short as a minute. While most of the light curves correspond to a low variability state of Sgr A*, the April 11 observations follow an X-ray flare and exhibit strongly enhanced variability. All of the light curves are consistent with a red-noise process, with a power spectral density (PSD) slope measured to be between −2 and −3 on timescales between 1 minute and several hours. Our results indicate a steepening of the PSD slope for timescales shorter than 0.3 hr. The spectral energy distribution is flat at 220 GHz, and there are no time lags between the 213 and 229 GHz frequency bands, suggesting low optical depth for the event horizon scale source. We characterize Sgr A*’s variability, highlighting the different behavior observed just after the X-ray flare, and use Gaussian process modeling to extract a decorrelation timescale and a PSD slope. We also investigate the systematic calibration uncertainties by analyzing data from independent data reduction pipelines.