The NANOGrav 15 yr Data Set: Constraints on Supermassive Black Hole Binaries from the Gravitational-wave Background
Authors: Agazie Gabriella , Anumarlapudi Akash , Archibald Anne M. , Baker Paul T. , Bécsy Bence , Blecha Laura , Bonilla Alexander , Brazier Adam , Brook Paul R. , Burke-Spolaor Sarah , Burnette Rand , Case Robin , Casey-Clyde J. Andrew , Charisi Maria , Chatterjee Shami , Chatziioannou Katerina , Cheeseboro Belinda D. , Chen Siyuan , Cohen Tyler , Cordes James M. , Cornish Neil J. , Crawford Fronefield , Cromartie H. Thankful , Crowter Kathryn , Cutler Curt J. , D’Orazio Daniel J. , DeCesar Megan E. , DeGan Dallas , Demorest Paul B. , Deng Heling , Dolch Timothy , Drachler Brendan , Ferrara Elizabeth C. , Fiore William , Fonseca Emmanuel , Freedman Gabriel E. , Gardiner Emiko , Garver-Daniels Nate , Gentile Peter A. , Gersbach Kyle A. , Glaser Joseph , Good Deborah C. , Gültekin Kayhan , Hazboun Jeffrey S. , Hourihane Sophie , Islo Kristina , Jennings Ross J. , Johnson Aaron , 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. , W. Lazio T. Joseph , Lewandowska Natalia , Littenberg Tyson B. , Liu Tingting , 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. , Petrov Polina , 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 , Schult Levi , Shapiro-Albert Brent J. , Siemens Xavier , Simon Joseph , Siwek Magdalena S. , Stairs Ingrid H. , Stinebring Daniel R. , Stovall Kevin , Sun Jerry P. , Susobhanan Abhimanyu , Swiggum Joseph K. , Taylor Jacob , Taylor Stephen R. , Turner Jacob E. , Unal Caner , Vallisneri Michele , Vigeland Sarah J. , Wachter Jeremy M. , Wahl Haley M. , Wang Qiaohong , Witt Caitlin A. , Wright David , Young Olivia ,   The NANOGrav Collaboration
Published: August 2023

Abstract
The NANOGrav 15 yr data set shows evidence for the presence of a low-frequency gravitational-wave background (GWB). While many physical processes can source such low-frequency gravitational waves, here we analyze the signal as coming from a population of supermassive black hole (SMBH) binaries distributed throughout the Universe. We show that astrophysically motivated models of SMBH binary populations are able to reproduce both the amplitude and shape of the observed low-frequency gravitational-wave spectrum. While multiple model variations are able to reproduce the GWB spectrum at our current measurement precision, our results highlight the importance of accurately modeling binary evolution for producing realistic GWB spectra. Additionally, while reasonable parameters are able to reproduce the 15 yr observations, the implied GWB amplitude necessitates either a large number of parameters to be at the edges of expected values or a small number of parameters to be notably different from standard expectations. While we are not yet able to definitively establish the origin of the inferred GWB signal, the consistency of the signal with astrophysical expectations offers a tantalizing prospect for confirming that SMBH binaries are able to form, reach subparsec separations, and eventually coalesce. As the significance grows over time, higher-order features of the GWB spectrum will definitively determine the nature of the GWB and allow for novel constraints on SMBH populations.