Arzoumanian, Z., Brazier, A., Burke-Spolaor, S., Chamberlin, S., Chatterjee, S., Christy, B., et al. (2016). The NANOGrav Nine-year Data Set: Limits on the Isotropic Stochastic Gravitational Wave Background. Astrophysical Journal, 821: 13, pp. 13. doi:10.3847/0004-637X/821/1/13.