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arxiv: 2306.16221 · v1 · pith:4ZJXYSAI · submitted 2023-06-28 · astro-ph.HE · gr-qc

The NANOGrav 15-year Data Set: Search for Anisotropy in the Gravitational-Wave Background

Gabriella Agazie , Akash Anumarlapudi , Anne M. Archibald , Zaven Arzoumanian , Paul T. Baker , Bence B\'ecsy , Laura Blecha , Adam Brazier
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Paul R. Brook Sarah Burke-Spolaor J. Andrew Casey-Clyde Maria Charisi Shami Chatterjee Tyler Cohen James M. Cordes Neil J. Cornish Fronefield Crawford H. Thankful Cromartie Kathryn Crowter Megan E. DeCesar Paul B. Demorest Timothy Dolch Brendan Drachler Elizabeth C. Ferrara William Fiore Emmanuel Fonseca Gabriel E. Freedman Emiko Gardiner Nate Garver-Daniels Peter A. Gentile Joseph Glaser Deborah C. Good Kayhan G\"ultekin Jeffrey S. Hazboun Ross J. Jennings Aaron D. Johnson Megan L. Jones Andrew R. Kaiser David L. Kaplan Luke Zoltan Kelley Matthew Kerr Joey S. Key Nima Laal Michael T. Lam William G. Lamb T. Joseph W. Lazio Natalia Lewandowska Tingting Liu Duncan R. Lorimer Jing Luo Ryan S. Lynch Chung-Pei Ma Dustin R. Madison Alexander McEwen James W. McKee Maura A. McLaughlin Natasha McMann Bradley W. Meyers Chiara M. F. Mingarelli Andrea Mitridate Cherry Ng David J. Nice Stella Koch Ocker Ken D. Olum Timothy T. Pennucci Benetge B. P. Perera Nihan S. Pol Henri A. Radovan Scott M. Ransom Paul S. Ray Joseph D. Romano Shashwat C. Sardesai Ann Schmiedekamp Carl Schmiedekamp Kai Schmitz Levi Schult Brent J. Shapiro-Albert Xavier Siemens Joseph Simon Magdalena S. Siwek Ingrid H. Stairs Daniel R. Stinebring Kevin Stovall Abhimanyu Susobhanan Joseph K. Swiggum Stephen R. Taylor Jacob E. Turner Caner Unal Michele Vallisneri Sarah J. Vigeland Haley M. Wahl Caitlin A. Witt Olivia Young (for the NANOGrav Collaboration)
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keywords anisotropybackgrounddatasetdistributionsmbhbsystemsdataevidence
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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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