tokens: ACKNOWLEDGEMENTS GBTrans activities are supported by a National Science Foundation (NSF) award AST-1516958. DRL was supported by NSF award number OIA-1458952. GG would like to thank Mayuresh Surnis and Dan Werthimer for useful discussions and Nate Garver-Daniels for the technical support for this work. GG, DRL, RP, and MAM are members of the NANOGrav Physics Frontiers Center that is supported by NSF award 1430284. SWE was supported by NSF award AST-1516170 to Virginia Tech. KR would like to acknowledge financial support from European Research Council Horizon 2020 grant (no. 694745). The Green Bank Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. We thank the referee for very helpful comments on an earlier version of the manuscript.
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