We thank the staff of the Arecibo Observatory and the Green Bank Observatory for their help with our observations. We also thank B. Adebahr, L. Connor, G. Desvignes, R. Eatough, R. Fender, M. Haverkorn, A. Karastergiou, R. Morganti, E. Petroff, F. Vieyro and J. Weisberg for suggestions and comments on the manuscript. The Arecibo Observatory is operated by SRI International under a cooperative agreement with the National Science Foundation (AST-1100968), and in alliance with Ana G. Méndez-Universidad Metropolitana and the Universities Space Research Association. The Green Bank Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Breakthrough Listen (BL) is managed by the Breakthrough Initiatives, sponsored by the Breakthrough Prize Foundation (http://www.breakthroughinitiatives.org). The research leading to these results received funding from the European Research Council (ERC) under the European Union’s Seventh Framework Programme (FP7/2007-2013). J.W.T.H. is a Netherlands Organisation for Scientific Research (NWO) Vidi Fellow and, together with D.M., K.G. and C.G.B., acknowledges funding for this work from ERC Starting Grant DRAGNET under contract number 337062. L.G.S. acknowledges financial support from the ERC Starting Grant BEACON under contract number 279702, as well as the Max Planck Society. A.M.A. is an NWO Veni Fellow. S.C., J.M.C., P.D., T.J.W.L., M.A.M. and S.M.R. are partially supported by the NANOGrav Physics Frontiers Center (NSF award 1430284). V.M.K. holds the Lorne Trottier Chair in Astrophysics & Cosmology and a Canada Research Chair and receives support from an NSERC Discovery Grant and Herzberg Prize, from an R. Howard Webster Foundation Fellowship from the Canadian Institute for Advanced Research (CIFAR), and from the FRQNT Centre de Recherche en Astrophysique du Québec. C.J.L. acknowledges support from NSF award 1611606. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. B.M. acknowledges support from the Spanish Ministerio de Economía y Competitividad (MINECO) under grants AYA2016-76012-C3-1-P and MDM-2014-0369 of ICCUB (Unidad de Excelencia ‘María de Maeztu’). S.M.R. is a CIFAR Senior Fellow. P.S. holds a Covington Fellowship at DRAO.