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The NANOGrav Collaboration receives support from National Science Foundation (NSF) Physics Frontiers Center award Nos. 1430284 and 2020265, the Gordon and Betty Moore Foundation, NSF AccelNet award No. 2114721, an NSERC Discovery Grant, and CIFAR. The Arecibo Observatory is a facility of the NSF operated under cooperative agreement (AST-1744119) by the University of Central Florida (UCF) in alliance with Universidad Ana G. Mendez (UAGM) and Yang Enterprises (YEI), Inc. The Green Bank Observatory is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc. The National Radio Astronomy Observatory is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc.

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

Publicated to:Astrophysical Journal Letters. 978 (2): L29- - 2025-01-10 978(2), DOI: 10.3847/2041-8213/ad99d3

Authors: Agazie, Gabriella; Anumarlapudi, Akash; Archibald, Anne M; Arzoumanian, Zaven; Baier, Jeremy G; Baker, Paul T; Becsy, 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; DeCesar, Megan E; Demorest, Paul B; Deng, Heling; Dey, Lankeswar; Dolch, Timothy; Esmyol, David; Ferrara, Elizabeth C; Fiore, William; Fonseca, Emmanuel; Freedman, Gabriel E; Gardiner, Emiko C; Garver-Daniels, Nate; Gentile, Peter A; Gersbach, Kyle A; Glaser, Joseph; Good, Deborah C; Gultekin, Kayhan; Hazboun, Jeffrey S; Jennings, Ross J; Johnson, Aaron D; Jones, Megan L; Kaplan, David L; Kelley, Luke Zoltan; Kerr, Matthew; Key, Joey S; Laal, Nima; Lam, Michael T; Lamb, William G; Larsen, Bjorn; Lazio, T Joseph W; Lewandowska, Natalia; dos Santos, Rafael R Lino; Liu, Tingting; Lorimer, Duncan R; 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; Ng, Cherry; Nice, David J; Ocker, Stella Koch; Olum, Ken D; Pennucci, Timothy T; Perera, Benetge B P; Pol, Nihan S; Radovan, Henri A; Ransom, Scott M; Ray, Paul S; Romano, Joseph D; Runnoe, Jessie C; Saffer, Alexander; Sardesai, Shashwat C; Schmiedekamp, Ann; Schmiedekamp, Carl; Schmitz, Kai; Shapiro-Albert, Brent J; Siemens, Xavier; Simon, Joseph; Siwek, Magdalena S; Fiscella, Sophia V Sosa; Stairs, Ingrid H; Stinebring, Daniel R; Stovall, Kevin; Susobhanan, Abhimanyu; Swiggum, Joseph K; Taylor, Stephen R; Turner, Jacob E; Unal, Caner; Vallisneri, Michele; van Haasteren, Rutger; Vigeland, Sarah J; von Eckardstein, Richard; Wahl, Haley M; Witt, Caitlin A; Wright, David; Young, Olivia

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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 gamma. 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, gamma(run)(f) = gamma + beta ln (f/f(ref)). 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 beta consistent with no running, beta is an element of[-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 beta. 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.

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