The NANOGrav 15 yr Data Set: Harmonic Analysis of the Pulsar Angular Correlations

Authors

• Gabriella Agazie, University of Wisconsin-Milwaukee
• Akash Anumarlapudi, University of Wisconsin-Milwaukee
• Anne M. Archibald, Newcastle University
• Zaven Arzoumanian, NASA Goddard Space Flight Center
• Jeremy G. Baier, Oregon State University
• Paul T. Baker, Widener University
• Bence Bécsy, Oregon State University
• Laura Blecha, University of Florida
• Kimberly K. Boddy, University of Texas at Austin
• Adam Brazier, Cornell Center for Astrophysics and Planetary Science
• Paul R. Brook, University of Birmingham
• Sarah Burke-Spolaor, West Virginia University
• Rand Burnette, Oregon State University
• J. Andrew Casey-Clyde, University of Connecticut
• Maria Charisi, Vanderbilt University
• Shami Chatterjee, Cornell Center for Astrophysics and Planetary Science
• Tyler Cohen, New Mexico Institute of Mining and Technology
• James M. Cordes, Cornell Center for Astrophysics and Planetary Science
• Neil J. Cornish, Montana State University
• Fronefield Crawford, Franklin and Marshall College, Lancaster
• H. Thankful Cromartie, National Academy of Sciences

Document Type

Article

Publication Title

Astrophysical Journal

Department

Physics

ISSN

15384357

Volume

985

Issue

1

DOI

10.3847/1538-4357/adc997

First Page

1

Last Page

12

Publication Date

5-16-2025

Abstract

Pulsar timing array observations have found evidence for an isotropic gravitational-wave background with the Hellings-Downs angular correlations between pulsar pairs. This interpretation hinges on the measured shape of the angular correlations, which is predominantly quadrupolar under general relativity. Here we explore a more flexible parameterization: we expand the angular correlations into a sum of Legendre polynomials and use a Bayesian analysis to constrain their coefficients with the 15 yr pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). When including Legendre polynomials with multipoles ℓ ≥ 2, we only find a significant signal in the quadrupole with an amplitude consistent with general relativity and nonzero at the ∼95% confidence level and a Bayes factor of 200. When we include multipoles ℓ ≤ 1, the Bayes factor evidence for quadrupole correlations decreases by more than an order of magnitude due to evidence for a monopolar signal at approximately 4 nHz, which has also been noted in previous analyses of the NANOGrav 15 yr data. Further work needs to be done in order to better characterize the properties of this monopolar signal and its effect on the evidence for quadrupolar angular correlations.

Creative Commons License

This work is licensed under a Creative Commons Attribution 4.0 International License.

Recommended Citation

Agazie, Gabriella; Anumarlapudi, Akash; Archibald, Anne M.; Arzoumanian, Zaven; Baier, Jeremy G.; Baker, Paul T.; Bécsy, Bence; Blecha, Laura; Boddy, Kimberly K.; Brazier, Adam; Brook, Paul R.; Burke-Spolaor, Sarah; Burnette, Rand; Casey-Clyde, J. Andrew; Charisi, Maria; Chatterjee, Shami; Cohen, Tyler; Cordes, James M.; Cornish, Neil J.; Crawford, Fronefield; and Cromartie, H. Thankful, "The NANOGrav 15 yr Data Set: Harmonic Analysis of the Pulsar Angular Correlations" (2025). Pacific Faculty Work. 108.
https://scholarlycommons.pacific.edu/all-faculty/108