The NANOGrav 15 yr Data Set: Running of the Spectral Index

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
• Adam Brazier, Cornell Center for Astrophysics and Planetary Science
• Paul R. Brook, University of Birmingham
• Sarah Burke-Spolaor, West Virginia 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
• Kathryn Crowter, The University of British Columbia
• Megan E. DeCesar, George Mason University

Document Type

Article

Publication Title

Astrophysical Journal Letters

Department

Physics

ISSN

20418213

Volume

978

Issue

2

DOI

10.3847/2041-8213/ad99d3

First Page

1

Last Page

14

Publication Date

1-6-2025

Abstract

The NANOGrav 15 yr data provide compelling evidence for a stochastic gravitational-wave (GW) background at nanohertz frequencies. The simplest model-independent approach to characterizing the frequency spectrum of this signal consists of a simple power-law fit involving two parameters: an amplitude A and a spectral index γ. In this Letter, we consider the next logical step beyond this minimal spectral model, allowing for a running (i.e., logarithmic frequency dependence) of the spectral index, grun (f ) = g + b ln (f /fref ). We fit this running-power-law (RPL) model to the NANOGrav 15 yr data and perform a Bayesian model comparison with the minimal constant-power-law (CPL) model, which results in a 95% credible interval for the parameter β consistent with no running, b Î [-0.80, 2.96], and an inconclusive Bayes factor, B(RPL versus CPL) = 0.69 ± 0.01. We thus conclude that, at present, the minimal CPL model still suffices to adequately describe the NANOGrav signal; however, future data sets may well lead to a measurement of nonzero β. Finally, we interpret the RPL model as a description of primordial GWs generated during cosmic inflation, which allows us to combine our results with upper limits from Big Bang nucleosynthesis, the cosmic microwave background, and LIGO–Virgo–KAGRA.

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; Brazier, Adam; Brook, Paul R.; 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; and DeCesar, Megan E., "The NANOGrav 15 yr Data Set: Running of the Spectral Index" (2025). Pacific Faculty Work. 110.
https://scholarlycommons.pacific.edu/all-faculty/110