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Lookup NU author(s): Dr Anne ArchibaldORCiD
This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).
© 2025. The Author(s)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.
Author(s): Agazie G, Anumarlapudi A, Archibald AM, Arzoumanian Z, Baier JG, Baker PT, Becsy B, Blecha L, Brazier A, Brook PR, Burke-Spolaor S, Casey-Clyde JA, Charisi M, Chatterjee S, Cohen T, Cordes JM, Cornish NJ, Crawford F, Cromartie HT, Crowter K, DeCesar ME, Demorest PB, Deng H, Dey L, Dolch T, Esmyol D, Ferrara EC, Fiore W, Fonseca E, Freedman GE, Gardiner EC, Garver-Daniels N, Gentile PA, Gersbach KA, Glaser J, Good DC, Gultekin K, Hazboun JS, Jennings RJ, Johnson AD, Jones ML, Kaplan DL, Kelley LZ, Kerr M, Key JS, Laal N, Lam MT, Lamb WG, Larsen B, Lazio JTW, Lewandowska N, Lino dos Santos RR, Liu T, Lorimer DR, Luo J, Lynch RS, Ma C-P, Madison DR, McEwen A, McKee JW, McLaughlin MA, McMann N, Meyers BW, Meyers PM, Mingarelli CMF, Mitridate A, Ng C, Nice DJ, Ocker SK, Olum KD, Pennucci TT, Perera BBP, Pol NS, Radovan HA, Ransom SM, Ray PS, Romano JD, Runnoe JC, Saffer A, Sardesai SC, Schmiedekamp A, Schmiedekamp C, Schmitz K, Schroder T, Shapiro-Albert BJ, Siemens X, Simon J, Siwek MS, Sosa Fiscella SV, Stairs IH, Stinebring DR, Stovall K, Susobhanan A, Swiggum JK, Taylor SR, Turner JE, Unal C, Vallisneri M, van Haasteren R, Vigeland SJ, von Eckardstein R, Wahl HM, Witt CA, Wright D, Young O
Publication type: Article
Publication status: Published
Journal: Astrophysical Journal Letters
Year: 2025
Volume: 978
Issue: 2
Print publication date: 10/01/2025
Online publication date: 06/01/2024
Acceptance date: 28/11/2024
Date deposited: 21/01/2025
ISSN (print): 2041-8205
ISSN (electronic): 2041-8213
Publisher: American Astronomical Society
URL: https://doi.org/10.3847/2041-8213/ad99d3
DOI: 10.3847/2041-8213/ad99d3
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