tokens: The authors would like to thank the anonymous referee whose comments improved the flow, precision, and clarity of the paper. It is a pleasure to thank Tony Piro, Dan Kasen, E. Sterl Phinney, Eliot Quataert, Maryam Modjaz, Jim Fuller, Lars Bildsten, Udi Nakar, Paul Duffell, and Luc Dessart for helpful discussions. A.Y.Q.H. is particularly grateful to Tony Piro and the community at Carnegie Observatories for their hospitality on Tuesdays during the period in which this work was performed. Thank you to the staff at the SMA, AMI, the VLA, Swift , and Chandra for rapidly scheduling and executing the observations. Thank you to David Palmer (LANL) for his assistance in searching the pointing data for Swift /BAT. Thank you to Michael J. Koss (Eureka Scientific Int), Andrew Drake (Caltech), Scott Adams (Caltech), Matt Hankins (Caltech), Kevin Burdge (Caltech), and Kirsty Taggart (LJMU) for assisting with optical spectroscopic observations. Thank you to Erik Petigura and David Hogg for their advice on figure aesthetics. Thank you to David Alexander Kann (IAA-CSIC) for pointing out an error in an early version of the paper posted to the arXiv. D.A.G. thanks Stéfan van der Walt and Ari Crellin-Quick for assistance with skyportal, which enabled the search for pre-explosion emission. A.Y.Q.H. is supported by a National Science Foundation Graduate Research Fellowship under grant No. DGE1144469. This work was supported by the GROWTH project funded by the National Science Foundation under PIRE grant No. 1545949. A.G.-Y. is supported by the EU via ERC grant No. 725161, the ISF, the BSF Transformative program, and by a Kimmel award. Y.T. studied as a GROWTH intern at Caltech during the summer and fall of 2017. C.C.N. thanks the funding from MOST grant 104-2923-M-008-004-MY5. R.L. is supported by a Marie Skłodowska-Curie Individual Fellowship within the Horizon 2020 European Union (EU) Framework Programme for Research and Innovation (H2020-MSCA-IF-2017-794467). A.H. acknowledges support by the I-Core Program of the Planning and Budgeting Committee and the Israel Science Foundation, and support by the ISF grant 647/18. This research was supported by a grant from the GIF, the German-Israeli Foundation for Scientific Research and Development. This research was funded in part by the Gordon and Betty Moore Foundation through Grant GBMF5076, and a grant from the Heising-Simons Foundation. A.C. acknowledges support from the NSF CAREER award N. 1455090 and from the NASA/ Chandra GI award N. GO8-19055A. Research support to I.A. is provided by the GROWTH project, funded by the National Science Foundation under grant No 1545949. This work is based on observations obtained with the Samuel Oschin Telescope 48 inch and the 60 inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. Major funding has been provided by the U.S. National Science Foundation under grant No. AST-1440341 and by the ZTF partner institutions: the California Institute of Technology, the Oskar Klein Centre, the Weizmann Institute of Science, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron, the University of Wisconsin–Milwaukee, and the TANGO Program of the University System of Taiwan. Partially based on observations made with the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. The Liverpool Telescope is operated by Liverpool John Moores University with financial support from the UK Science and Technology Facilities Council. LT is located on the island of La Palma, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. The scientific results reported in this article are based in part on observations made by the Chandra X-ray Observatory . The data presented here were obtained in part with ALFOSC, which is provided by the Instituto de Astrofisica de Andalucia (IAA) under a joint agreement with the University of Copenhagen and NOTSA. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. We acknowledge the support of the staff of the Xinglong 2.16 m telescope. This work is supported by the National Natural Science Foundation of China (NSFC grants 11325313 and 11633002), and the National Program on Key Research and Development Project (grant No. 2016YFA0400803). SED Machine is based upon work supported by the National Science Foundation under grant No. 1106171. This publication has made use of data collected at Lulin Observatory, partly supported by MoST grant 105-2112-M-008-024-MY3. The JEKYLL simulations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at Parallelldatorcentrum (PDC). Facilities: CFHT , Keck:I (LRIS) , Hale (DBSP) , AMI , Liverpool:2 m (IO:O , SPRAT) , DCT , Swift (UVOT , XRT) , Beijing:2.16 m , EVLA , SMA , LO:1 m , NOT (ALFOSC). Software: Astropy (Astropy Collaboration et al. 2013 , 2018 ), IPython (Pérez Granger 2007 ), matplotlib (Hunter 2007 ), numpy (Oliphant 2006 ), scipy (Jones et al. 2001 ), extinction (Barbary 2016 ) SkyPortal (Van der Walt et al. 2019 ).
ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O B-Person I-Person B-Person I-Person B-Person I-Person I-Person B-Person I-Person B-Person I-Person B-Person I-Person B-Person I-Person B-Person I-Person B-Person I-Person O B-Person I-Person O O O B-Person O O O O B-Person I-Person O O O O B-Observatory I-Observatory O O O O O O O O O O O O O O O O O O O O O B-Telescope B-Telescope O B-Telescope B-Observatory O O B-Telescope O O O O O O O O O O B-Person I-Person B-Organization O O O O O O O O O B-Observatory B-Telescope O O O B-Person I-Person I-Person B-Organization I-Organization I-Organization B-Person I-Person B-Organization B-Person I-Person B-Organization B-Person I-Person B-Organization B-Person I-Person B-Organization O B-Person I-Person B-Organization O O O B-Wavelength O O O O O B-Person I-Person O B-Person I-Person O O O O O O O O O B-Person I-Person I-Person B-Organization O O O O O O O O O O O O O O O B-Archive B-Person O B-Person I-Person I-Person I-Person O B-Person I-Person O O O B-Software O O O O O O O B-Person O O O O B-Organization I-Organization I-Organization B-Fellowship I-Fellowship I-Fellowship O B-Fellowship I-Fellowship I-Fellowship O O O O O O B-Grant I-Grant O O O B-Organization I-Organization I-Organization O B-Grant I-Grant I-Grant I-Grant B-Person O O O O B-Location O B-Organization B-Grant I-Grant I-Grant O B-Organization O B-Grant I-Grant I-Grant O O O B-Grant I-Grant B-Person O O O B-Fellowship I-Fellowship O B-Organization O O O O O O O B-Person O O O O B-Organization B-Grant I-Grant B-Person O O O O B-Fellowship I-Fellowship I-Fellowship I-Fellowship O O B-Fellowship I-Fellowship B-Location I-Location B-Location B-Grant I-Grant I-Grant I-Grant I-Grant I-Grant B-Grant B-Person O O O O B-Grant I-Grant O O B-Organization I-Organization I-Organization I-Organization O O B-Organization I-Organization I-Organization O O O O B-Organization B-Grant I-Grant O O O O O O O O O B-Organization O B-Organization I-Organization I-Organization I-Organization I-Organization I-Organization I-Organization O O O O O O O O B-Organization I-Organization I-Organization I-Organization I-Organization O B-Grant I-Grant O O O O O B-Organization I-Organization B-Person O O O O B-Organization B-Grant I-Grant I-Grant I-Grant O O O B-Organization B-Grant I-Grant I-Grant I-Grant I-Grant O O O B-Person O O O O B-Grant I-Grant O O O B-Organization I-Organization I-Organization O B-Grant I-Grant I-Grant O O O O O O O O O B-Telescope I-Telescope I-Telescope I-Telescope I-Telescope O O B-Telescope I-Telescope I-Telescope O O B-Observatory I-Observatory O O O O B-Survey I-Survey I-Survey O O O O O O O O B-Location B-Organization I-Organization I-Organization O B-Grant I-Grant I-Grant O O O B-Survey O O O B-Organization I-Organization I-Organization I-Organization O B-Organization I-Organization I-Organization O B-Organization I-Organization I-Organization I-Organization O B-Organization I-Organization I-Organization O B-Organization I-Organization I-Organization B-Instrument I-Instrument O B-Organization I-Organization I-Organization O O B-Grant I-Grant O O B-Organization I-Organization I-Organization I-Organization O O O O O O O B-Telescope I-Telescope I-Telescope O O O B-Organization I-Organization I-Organization I-Organization I-Organization O O B-Observatory I-Observatory I-Observatory I-Observatory I-Observatory I-Observatory B-Location I-Location I-Location O O B-Organization I-Organization I-Organization I-Organization I-Organization O B-Telescope I-Telescope O O O B-Organization I-Organization I-Organization I-Organization O O O O O B-Location B-Organization I-Organization I-Organization I-Organization I-Organization B-Person O O O O B-Location I-Location I-Location I-Location O O B-Location B-Observatory I-Observatory I-Observatory I-Observatory I-Observatory I-Observatory O O B-Organization I-Organization I-Organization I-Organization I-Organization O O O O O O O O O O O O O O O O B-Telescope I-Telescope I-Telescope O O O O O O O O O O B-Instrument O O O O O B-Organization I-Organization I-Organization I-Organization I-Organization B-Organization O O O O O O B-Organization I-Organization I-Organization O B-Organization O B-Telescope I-Telescope O O O O O O B-Observatory I-Observatory I-Observatory O O B-Organization I-Organization I-Organization I-Organization I-Organization I-Organization I-Organization O O O O O B-Organization I-Organization O O B-Organization I-Organization O O O O O O O O O B-Telescope I-Telescope I-Telescope I-Telescope O O O O O O B-Organization I-Organization I-Organization I-Organization I-Organization I-Organization B-Organization B-Grant I-Grant O B-Grant O O B-Grant I-Grant I-Grant I-Grant I-Grant I-Grant I-Grant I-Grant B-Grant I-Grant I-Grant B-Software I-Software O O O O O O O B-Organization I-Organization I-Organization O B-Grant I-Grant I-Grant O O O O O O O O O B-Observatory I-Observatory O O O B-Organization B-Grant I-Grant O B-Model I-Model O O O O O O O B-ComputingFacility I-ComputingFacility I-ComputingFacility I-ComputingFacility I-ComputingFacility B-ComputingFacility B-ComputingFacility I-ComputingFacility B-ComputingFacility O B-Telescope O B-Observatory B-Instrument O B-Telescope B-Instrument O B-Telescope O B-Observatory B-Telescope B-Instrument O B-Instrument O B-Telescope O B-Observatory B-Telescope O B-Telescope O B-Observatory B-Telescope O B-Telescope O B-Telescope O B-Observatory B-Telescope O B-Telescope B-Instrument O B-Software B-Citation I-Citation I-Citation I-Citation I-Citation O B-Citation O B-Software B-Citation I-Citation I-Citation O B-Software B-Citation I-Citation O B-Software B-Citation I-Citation O B-Software B-Citation I-Citation I-Citation I-Citation O B-Software B-Citation I-Citation O B-Software B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O