Akademik Veri Yönetim Sistemi
ASTROPHYSICAL JOURNAL, cilt.978, sa.1, 2025 (SCI-Expanded, Scopus)
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 (PTAs) through excursions from, and breaks in, the expected fGW-2/3 power law of the GWB strain spectrum. To do this, we create a semianalytic SMBHB population model, fit to North American Nanohertz Observatory for Gravitational Waves (NANOGrav's) 15 yr GWB amplitude, and with 1000 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 a p-value significance p = 0.05-0.06 (approximate to 1.8 sigma-1.9 sigma). The second, at 16 nHz, is above our GWB realizations with p = 0.04-0.15 (approximate to 1.4 sigma-2.1 sigma). We explore the properties of a loud SMBHB that could cause such an excursion. Our simulations also show that the expected number of SMBHBs decreases by 3 orders of magnitude, from similar to 106 to similar to 103, between 2 and 20 nHz. This causes a break in the strain spectrum as the stochasticity of the background breaks down at 26-19+28nHz , 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.