Astrophysics Milestones for PTAs

The NANOGrav Collaboration found strong Bayesian evidence for a common-spectrum stochastic process in its 12.5-yr pulsar timing array (PTA) dataset. However, evidence for the quadrupolar Hellings \& Downs interpulsar correlations was not yet significant. We emulate and extend the NANOGrav dataset, injecting a wide range of stochastic gravitational wave background (GWB) signals that encompass a variety of amplitudes and spectral shapes. We then apply our standard detection pipeline and explore three key astrophysical milestones: (I) robust detection of the GWB; (II) determination of the source of the GWB; and (III) measurement of the properties of the GWB spectrum. Given the amplitude measured in the 12.5 yr analysis and assuming this signal is a GWB, we expect to accumulate robust evidence of an interpulsar-correlated GWB signal with 15--17 yrs of data, i.e., an additional 2-5 yrs from the 12.5 yr dataset. At the initial detection, the achieved fractional uncertainty should be sufficient to distinguish a SMBHB source for the background from other, more exotic, sources. We also show that it is possible to distinguish power-law models from those with a spectral turn-over with 20 yrs of data. Finally, with the IPTA data combination, these milestones will be achieved much earlier.