The NANOGrav 15 yr Data Set: Looking for Signs of Discreteness in the Gravitational-wave Background

The cosmic merger history of supermassive black hole binaries (SMBHBs) is expected to produce a low-frequency gravitational wave background (GWB). Here we investigate how signs of the discrete nature of this GWB can manifest in pulsar timing arrays through excursions from, and breaks in, the expected f_GW^-2/3 power-law of the GWB strain spectrum. To do this, we create a semi-analytic SMBHB population model, fit to NANOGrav's 15 yr GWB amplitude, and with 1,000 realizations we study the populations' characteristic strain and residual spectra. Comparing our models to the NANOGrav 15 yr spectrum, we find two interesting excursions from the power-law. The first, at 2 nHz, is below our GWB realizations with p-value significance p = 0.05 to 0.06 (≈ 1.8 σ - 1.9 σ). The second, at 16 nHz, is above our GWB realizations with p = 0.04 to 0.15 (≈ 1.4 σ - 2.1 σ). We explore the properties of a loud SMBHB which could cause such an excursion. Our simulations also show that the expected number of SMBHBs decreases by three orders of magnitude, from ∼ 10^6 to ∼ 10^3, between 2 nHz and 20 nHz. This causes a break in the strain spectrum as the stochasticity of the background breaks down at 26^+28_-19 nHz, consistent with predictions pre-dating GWB measurements. The diminished GWB signal from SMBHBs at frequencies above the 26 nHz break opens a window for PTAs to detect continuous GWs from individual SMBHBs or GWs from the early universe.