Disentangling Multiple Astrophysical Background Sources in PTA Datasets

With the strong evidence of a common-spectrum stochastic process in the NANOGrav Collaboration’s 12.5-yr dataset, it is crucial to assess the effects of the plethora of astrophysical sources that could contribute to the stochastic gravitational wave background (GWB). Using the same dataset creation and injection techniques as in Pol et al. (2021), we assess the separability of multiple GWBs by creating single and multiple source datasets. We investigate these injected sources using typical pulsar timing array (PTA) analysis techniques to assess recovery and separation of individual astrophysical backgrounds. We find that for a moderately strong underlying GWB on top of a GWB generated by primordial gravitational waves and supermassive black hole binaries, respectively (Ω_PGW/Ω_SMBHB = 0.5), PTAs begin accumulating evidence for the additional source after around 17 years of data. At 20 years of data, while the amplitude uncertainty region is still fairly large, we are able to constrain the spectral index of the weaker GWB to a fractional uncertainty of 40%. Using these methods and findings, we outline a basic protocol to search for multiple backgrounds in future PTA datasets.