Exploring Realistic Nanohertz Gravitational-wave Backgrounds

Hundreds of millions supermassive black hole binaries are expected to contribute the gravitational-wave signal in nanohertz frequency band. Their is often approximated either as an isotropic Gaussian stochastic background with a power-law spectrum, or individual source corresponding brightest binary. In reality, best described combination and few modeled individually. We present method that uses this approach efficiently create realistic pulsar timing array datasets using synthetic catalogs based on Illustris cosmological hydrodynamic simulation. explore three different properties such backgrounds which could help distinguish them from those formed early universe: i) their characteristic strain spectrum; ii) statistical isotropy; iii) variance spatial correlations. also investigate how presence confusion noise affects detection prospects binaries. calculate signal-to-noise ratios realizations for simulated NANOGrav 12.5-year dataset extended time span 15 years. find $\sim$6% produce systems larger than 5, suggesting might soon be detected (the fraction increases $\sim$41% at 20 years). These can taken pessimistic prediction upcoming 15-year dataset, since it does not include effect potentially improved solutions newly added pulsars.