The NANOGrav 15-year Data Set: Search for Anisotropy in the Gravitational-Wave Background
Authors: Gabriella Agazie (for the NANOGrav Collaboration), Akash Anumarlapudi (for the NANOGrav Collaboration), Anne M. Archibald (for the NANOGrav Collaboration), Zaven Arzoumanian (for the NANOGrav Collaboration), Paul T. Baker (for the NANOGrav Collaboration), Bence Bécsy (for the NANOGrav Collaboration), Laura Blecha (for the NANOGrav Collaboration), Adam Brazier (for the NANOGrav Collaboration), Paul R. Brook (for the NANOGrav Collaboration), Sarah Burke-Spolaor (for the NANOGrav Collaboration), J. Andrew Casey-Clyde (for the NANOGrav Collaboration), Maria Charisi (for the NANOGrav Collaboration), Shami Chatterjee (for the NANOGrav Collaboration), Tyler Cohen (for the NANOGrav Collaboration), James M. Cordes (for the NANOGrav Collaboration), Neil J. Cornish (for the NANOGrav Collaboration), Fronefield Crawford (for the NANOGrav Collaboration), H. Thankful Cromartie (for the NANOGrav Collaboration), Kathryn Crowter (for the NANOGrav Collaboration), Megan E. DeCesar (for the NANOGrav Collaboration), Paul B. Demorest (for the NANOGrav Collaboration), Timothy Dolch (for the NANOGrav Collaboration), Brendan Drachler (for the NANOGrav Collaboration), Elizabeth C. Ferrara (for the NANOGrav Collaboration), William Fiore (for the NANOGrav Collaboration), Emmanuel Fonseca (for the NANOGrav Collaboration), Gabriel E. Freedman (for the NANOGrav Collaboration), Emiko Gardiner (for the NANOGrav Collaboration), Nate Garver-Daniels (for the NANOGrav Collaboration), Peter A. Gentile (for the NANOGrav Collaboration), Joseph Glaser (for the NANOGrav Collaboration), Deborah C. Good (for the NANOGrav Collaboration), Kayhan Gültekin (for the NANOGrav Collaboration), Jeffrey S. Hazboun (for the NANOGrav Collaboration), Ross J. Jennings (for the NANOGrav Collaboration), Aaron D. Johnson (for the NANOGrav Collaboration), Megan L. Jones (for the NANOGrav Collaboration), Andrew R. Kaiser (for the NANOGrav Collaboration), David L. Kaplan (for the NANOGrav Collaboration), Luke Zoltan Kelley (for the NANOGrav Collaboration), Matthew Kerr (for the NANOGrav Collaboration), Joey S. Key (for the NANOGrav Collaboration), Nima Laal (for the NANOGrav Collaboration), Michael T. Lam (for the NANOGrav Collaboration), William G. Lamb (for the NANOGrav Collaboration), T. Joseph W. Lazio (for the NANOGrav Collaboration), Natalia Lewandowska (for the NANOGrav Collaboration), Tingting Liu (for the NANOGrav Collaboration), Duncan R. Lorimer (for the NANOGrav Collaboration), Jing Luo (for the NANOGrav Collaboration), Ryan S. Lynch (for the NANOGrav Collaboration), Chung-Pei Ma (for the NANOGrav Collaboration), Dustin R. Madison (for the NANOGrav Collaboration), Alexander McEwen (for the NANOGrav Collaboration), James W. McKee (for the NANOGrav Collaboration), Maura A. McLaughlin (for the NANOGrav Collaboration), Natasha McMann (for the NANOGrav Collaboration), Bradley W. Meyers (for the NANOGrav Collaboration), Chiara M. F. Mingarelli (for the NANOGrav Collaboration), Andrea Mitridate (for the NANOGrav Collaboration), Cherry Ng (for the NANOGrav Collaboration), David J. Nice (for the NANOGrav Collaboration), Stella Koch Ocker (for the NANOGrav Collaboration), Ken D. Olum (for the NANOGrav Collaboration), Timothy T. Pennucci (for the NANOGrav Collaboration), Benetge B. P. Perera (for the NANOGrav Collaboration), Nihan S. Pol (for the NANOGrav Collaboration), Henri A. Radovan (for the NANOGrav Collaboration), Scott M. Ransom (for the NANOGrav Collaboration), Paul S. Ray (for the NANOGrav Collaboration), Joseph D. Romano (for the NANOGrav Collaboration), Shashwat C. Sardesai (for the NANOGrav Collaboration), Ann Schmiedekamp (for the NANOGrav Collaboration), Carl Schmiedekamp (for the NANOGrav Collaboration), Kai Schmitz (for the NANOGrav Collaboration), Levi Schult (for the NANOGrav Collaboration), Brent J. Shapiro-Albert (for the NANOGrav Collaboration), Xavier Siemens (for the NANOGrav Collaboration), Joseph Simon (for the NANOGrav Collaboration), Magdalena S. Siwek (for the NANOGrav Collaboration), Ingrid H. Stairs (for the NANOGrav Collaboration), Daniel R. Stinebring (for the NANOGrav Collaboration), Kevin Stovall (for the NANOGrav Collaboration), Abhimanyu Susobhanan (for the NANOGrav Collaboration), Joseph K. Swiggum (for the NANOGrav Collaboration), Stephen R. Taylor (for the NANOGrav Collaboration), Jacob E. Turner (for the NANOGrav Collaboration), Caner Unal (for the NANOGrav Collaboration), Michele Vallisneri (for the NANOGrav Collaboration), Sarah J. Vigeland (for the NANOGrav Collaboration), Haley M. Wahl (for the NANOGrav Collaboration), Caitlin A. Witt (for the NANOGrav Collaboration), Olivia Young (for the NANOGrav Collaboration)
Abstract: The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has reported evidence for the presence of an isotropic nanohertz gravitational wave background (GWB) in its 15 yr dataset. However, if the GWB is produced by a population of inspiraling supermassive black hole binary (SMBHB) systems, then the background is predicted to be anisotropic, depending on the distribution of these systems in the local Universe and the statistical properties of the SMBHB population. In this work, we search for anisotropy in the GWB using multiple methods and bases to describe the distribution of the GWB power on the sky. We do not find significant evidence of anisotropy, and place a Bayesian $95\%$ upper limit on the level of broadband anisotropy such that $(C_{l>0} / C_{l=0}) < 20\%$. We also derive conservative estimates on the anisotropy expected from a random distribution of SMBHB systems using astrophysical simulations conditioned on the isotropic GWB inferred in the 15-yr dataset, and show that this dataset has sufficient sensitivity to probe a large fraction of the predicted level of anisotropy. We end by highlighting the opportunities and challenges in searching for anisotropy in pulsar timing array data.
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