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tokens: We thank the referee for useful suggestions and comments that improved the quality of this manuscript. The NANOGrav project receives support from NSF Physics Frontier Center award number 1430284. NANOGrav research at UBC is supported by an NSERC Discovery Grant and Discovery Accelerator Supplement and by the Canadian Institute for Advanced Research. We thank our colleagues in the International Pulsar Timing Array for comments and useful discussions. We thank Alberto Sesana for commenting on our astrophysical modeling and interpretation. M.V. and J.S. acknowledge support from the JPL RTD program. Portions of this research were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. S.R.T. was partially supported by an appointment to the NASA Postdoctoral Program at the Jet Propulsion Laboratory, administered by Oak Ridge Associated Universities through a contract with NASA. S.R.T. thanks E.R.S. for fruitful discussions. J.A.E. was partially supported by NASA through Einstein Fellowship grant PF4-150120. S.B.S. was supported by NSF award #1458952. P.T.B. acknowledges support from the West Virginia University Center for Gravitational Waves and Cosmology. M.A.M. was partially supported by NSF award OIA-1458952. W.W.Z. is supported by the CAS Pioneer Hundred Talents Program and the Strategic Priority Research Program of the Chinese Academy of Sciences grant No. XDB23000000. R.v.H. was supported by NASA Einstein Fellowship grant PF3-140116. This work was supported in part by National Science Foundation grant No. PHYS-1066293 and by the hospitality of the Aspen Center for Physics. Portions of this work performed at NRL are supported by the Chief of Naval Research. This research was performed in part using the Zwicky computer cluster at Caltech supported by NSF under MRI-R2 award No. PHY-0960291 and by the Sherman Fairchild Foundation. A majority of the computational work was performed on the Nemo cluster at UWM supported by NSF grant No. 0923409. Parts of the analysis in this work were carried out on the Nimrod cluster made available by S.M.R. Data for this project were collected using the facilities of the National Radio Astronomy Observatory and the Arecibo Observatory. The National Radio Astronomy Observatory is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc. The Green Bank Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The Arecibo Observatory is operated by SRI International under a cooperative agreement with the NSF (AST-1100968), and in alliance with Ana G. Méndez-Universidad Metropolitana and the Universities Space Research Association. This research is part of the Blue Waters sustained-petascale computing project, which is supported by the National Science Foundation (awards OCI-0725070 and ACI-1238993) and the state of Illinois. Blue Waters is a joint effort of the University of Illinois at Urbana-Champaign and its National Center for Supercomputing Applications. Some of the algorithms used in this article were optimized using the Blue Waters allocation “Accelerating the detection of gravitational waves with GPUs.” The Flatiron Institute is supported by the Simons Foundation.