Mansi M. Kasliwal³, Marat Gilfanov^10,11, Simone Garrappa^1,2, Vaidehi S. Paliya¹², Tomás Ahumada¹³, Shreya Anand³, Cristina Barbarino¹⁴, Eric C. Bellm¹⁵, Valéry Brinnel², Sara Buson¹⁶, S. Bradley Cenko^17,18, Michael W. Coughlin¹⁹, Kishalay De³, Richard Dekany²⁰, Sara Frederick¹³, Avishay Gal-Yam²¹, Suvi Gezari^22,13, Marcello Giroletti²³, Matthew J. Graham³, Viraj Karambelkar³, Shigeo S. Kimura²⁴, Albert K. H. Kong²⁵, Erik C. Kool¹⁴, Russ R. Laher²⁶, Pavel Medvedev¹⁰, Jannis Necker^1,2, Jakob Nordin², Daniel A. Perley²⁷, Mickael Rigault²⁸, Ben Rusholme²⁶, Steve Schulze²⁹, Tassilo Schweyer¹⁴, Leo P. Singer³⁰, Jesper Sollerman¹⁴, Nora Linn Strotjohann²¹, Rashid Sunyaev^10,11, Jakob van Santen¹, Richard Walters²⁰, B. Theodore Zhang⁸, and Erez Zimmerman²¹

• ¹Deutsches Elektronen Synchrotron DESY, Platanenallee 6, D-15738 Zeuthen, Germany
• ²Institut für Physik, Humboldt-Universität zu Berlin, D-12489 Berlin, Germany
• ³Division of Physics, Mathematics, and Astronomy, California Institute of Technology, 1200 East California Boulevard, MC 249-17, Pasadena, California 91125, USA
• ⁴Leiden Observatory, Leiden University, Postbus 9513, 2300 RA, Leiden, Netherlands
• ⁵Fakultät für Physik und Astronomie, Ruhr-Universität Bochum, D-44780 Bochum, Germany
• ⁶Department of Physics, Arizona State University, Tempe, Arizona 85287-1504, USA
• ⁷Department of Physics; Department of Astronomy and Astrophysics; Center for Multimessenger Astrophysics, Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
• ⁸Center for Gravitational Physics, Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto, Kyoto 606-8502, Japan
• ⁹International Centre for Radio Astronomy Research - Curtin University, GPO Box U1987, Perth, Washington 6845, Australia
• ¹⁰Space Research Institute (IKI), Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow 117997, Russia
• ¹¹Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85741 Garching, Germany
• ¹²Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital-263001 Uttarakhand, India
• ¹³Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA
• ¹⁴Department of Astronomy, The Oskar Klein Centre, Stockholm University, SE-106 91 Stockholm, Sweden
• ¹⁵DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, Washington 98195, USA
• ¹⁶Institut für Theoretische Physik and Astrophysik, Universität Würzburg, D-97074 Würzburg, Germany
• ¹⁷Astrophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, Maryland 20771, USA
• ¹⁸Joint Space-Science Institute, University of Maryland, College Park, Maryland 20742, USA
• ¹⁹School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA
• ²⁰Caltech Optical Observatories, California Institute of Technology, 1200 East California Boulevard, MC 249-17, Pasadena, California 91125, USA
• ²¹Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 234 Herzl Street 76100 Rehovot, Israel
• ²²Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA
• ²³INAF - Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, I-40129 Bologna, Italy
• ²⁴Frontier Research Institute for Interdisciplinary Sciences; Astronomical Institute, Tohoku University, Sendai 980-8574, Japan
• ²⁵Institute of Astronomy, National Tsing Hua University, No. 101 Section 2 Kuang-Fu Road, Hsinchu 30013, Taiwan
• ²⁶IPAC, California Institute of Technology, 1200 East California Boulevard, MC 249-17, Pasadena, California 91125, USA
• ²⁷Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, United Kingdom
• ²⁸Univ Lyon, Univ Claude Bernard Lyon 1, CNRS/IN2P3, UMR 5822, F-69622 Villeurbanne, France
• ²⁹The Oskar Klein Centre, Physics Department of Physics, Stockholm University, Albanova University Center, SE-106 91 Stockholm, Sweden
• ³⁰NASA Goddard Space Flight Center, University of Maryland, Baltimore County, Greenbelt, Maryland 20771, USA

• ^*Corresponding author. simeon.reusch@desy.de.
• ^†Corresponding author. marek.kowalski@desy.de