tokens: Kohta Murase thanks Poonam Chandra, Boaz Katz, Eran Ofek, Todd Thompson, and Eli Waxman for useful discussions. We also thank internal referees in the Fermi -LAT Collaboration, especially Regina Caputo, Seth Digel, Nicola Omodei, and Nicolas Renault-Tinacci for reviewing the manuscript. The work of Kohta Murase is supported by NSF grant No. PHY-1620777 and the Alfred P. Sloan Foundation. He also acknowledges the Hubble Fellowship through NASA and STScI during the early stages of this work. A.F. was supported by the Initiative and Networking Fund of the Helmholtz Association. Keiichi Maeda acknowledges support by JSPS KAKENHI grant Nos. 18H04585, 18H05223, 17H02864. J.F.B. is supported by NSF grant No. PHY-1714479. The preliminary estimates and results were presented at the PCTS workshop at Princeton University in 2015, the AMON workshop at Penn State University in 2015, and the supernova workshop at ISSI in 2016. The Fermi -LAT Collaboration acknowledges generous ongoing support from a number of agencies and institutes that have supported both the development and the operation of the LAT, as well as scientific data analysis. These include the National Aeronautics and Space Administration and the Department of Energy in the United States; the Commissariat à l’Energie Atomique and the Centre National de la Recherche Scientifique Institut National de Physique Nucléaire et de Physique des Particules in France; the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy; the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization (KEK), and Japan Aerospace Exploration Agency (JAXA) in Japan; and the K. A. Wallenberg Foundation, the Swedish Research Council, and the Swedish National Space Board in Sweden. Additional support for science analysis during the operations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy and the Centre National d’Études Spatiales in France. This work was performed in part under DOE Contract DE-AC02-76SF00515.
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