tokens: R.L. thanks Andrew Wetzler, Wen-fai Fong, Mark Sullivan, Dan Milisavljevic, Giorgos Leloudas, Jesper Sollerman, and Ryan Chornock for useful discussions and acknowledges helpful interactions with Lars Bildsten, Eliot Quataert, and Dan Kasen at a PTF Theory Network retreat funded by the Gordon and Betty Moore Foundation through Grant GBMF5076. We thank J. Silverman, B. Dilday, J. Bloom, B. Sesar, D. Levitan, P. Groot, D. Perley, A. Horesh, K. Mooley, and D. Xu for assisting with the observations presented in this paper. The Intermediate Palomar Transient Factory project is a scientific collaboration among the California Institute of Technology, Los Alamos National Laboratory, the University of Wisconsin–Milwaukee, the Oskar Klein Center, the Weizmann Institute of Science, the TANGO Program of the University System of Taiwan, and the Kavli Institute for the Physics and Mathematics of the Universe. Support for HST Program GO-13864 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the AURA, Inc., under NASA contract NAS 5-26555. We thank F. Yuan, M. Sullivan, D. Perley, R. M. Quimby, and S. B. Cenko for their contributions to the HST proposal. This work was supported by the GROWTH project funded by the National Science Foundation under Grant 1545949. The National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, provided staff, computational resources, and data storage for this project. A.G.-Y. is supported by the EU/FP7 via ERC grant No. 307260, the Quantum Universe I-Core program by the Israeli Committee for planning and funding, and the ISF, Minerva and ISF grants, WIS-UK “making connections,” and Kimmel and YeS awards. A.V.F. is grateful for financial support from the Christopher R. Redlich Fund, the TABASGO Foundation, and NSF grant AST-1211916. D.A.H. and C.M. are supported by NSF grant AST-313484. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facilities: PO:1.2m, PO:1.5m, PO:Hale, Keck:I, Keck:II, Subaru, HST .
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