The NANOGrav 15 yr Data Set: Looking for Signs of Discreteness in the Gravitational-wave Background
Authors: Agazie Gabriella , Anumarlapudi Akash , Archibald Anne M. , Arzoumanian Zaven , Baier Jeremy George , Baker Paul T. , Bécsy Bence , Blecha Laura , Brazier Adam , Brook Paul R. , Brown Lucas , Burke-Spolaor Sarah , Casey-Clyde J. Andrew , Charisi Maria , Chatterjee Shami , Cohen Tyler , Cordes James M. , Cornish Neil J. , Crawford Fronefield , Cromartie H. Thankful , Crowter Kathryn , DeCesar Megan E. , Demorest Paul B. , Deng Heling , Dolch Timothy , Ferrara Elizabeth C. , Fiore William , Fonseca Emmanuel , Freedman Gabriel E. , Garver-Daniels Nate , Gentile Peter A. , Glaser Joseph , Good Deborah C. , Gültekin Kayhan , Hazboun Jeffrey S. , Jennings Ross J. , Johnson Aaron D. , Jones Megan L. , Kaiser Andrew R. , Kaplan David L. , Kelley Luke Zoltan , Kerr Matthew , Key Joey S. , Laal Nima , Lam Michael T. , Lamb William G. , Larsen Bjorn , Lazio T. Joseph W. , Lewandowska Natalia , Liu Tingting , Lorimer Duncan R. , Luo Jing , Lynch Ryan S. , Ma Chung-Pei , Madison Dustin R. , McEwen Alexander , McKee James W. , McLaughlin Maura A. , McMann Natasha , Meyers Bradley W. , Meyers Patrick M. , Mingarelli Chiara M. F. , Mitridate Andrea , Natarajan Priyamvada , Ng Cherry , Nice David J. , Ocker Stella Koch , Olum Ken D. , Pennucci Timothy T. , Perera Benetge B. P. , Pol Nihan S. , Radovan Henri A. , Ransom Scott M. , Ray Paul S. , Romano Joseph D. , Runnoe Jessie C. , Sardesai Shashwat C. , Schmiedekamp Ann , Schmiedekamp Carl , Schmitz Kai , Shapiro-Albert Brent J. , Siemens Xavier , Simon Joseph , Siwek Magdalena S. , Sosa Fiscella Sophia V. , Stairs Ingrid H. , Stinebring Daniel R. , Stovall Kevin , Susobhanan Abhimanyu , Swiggum Joseph K. , Taylor Stephen R. , Turner Jacob E. , Unal Caner , Vallisneri Michele , Vigeland Sarah J. , Wahl Haley M. , Willson London , Witt Caitlin A. , Wright David , Young Olivia 
Published: December 2024

Abstract
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

f

GW

−
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 (≈1.8σ–1.9σ). The second, at 16 nHz, is above our GWB realizations with p = 0.04–0.15 (≈1.4σ–2.1σ). 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 ∼106 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

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19

+
28

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.