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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).
© 2026. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the https://creativecommons.org/licenses/by/4.0/. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. The NANOGrav 15 yr (NG15) data set provides evidence for a gravitational-wave background (GWB) signal at nHz frequencies, which is expected to originate either from a cosmic population of inspiraling supermassive black hole binaries or new particle physics in the early Universe. A firm identification of the source of the NG15 signal requires an accurate reconstruction of its frequency spectrum. In this Letter, we provide such a spectral characterization of the NG15 signal based on a piecewise power-law (PPL) ansatz that strikes a balance between existing alternatives in the literature. Our PPL reconstruction is more flexible than the standard constant power-law model, which describes the GWB spectrum in terms of only two parameters: an amplitude A and a spectral index γ. Concurrently, it better approximates physically realistic GWB spectra—especially those of cosmological origin—than the free spectral model, since the latter allows for arbitrary variations in the GWB amplitude from one frequency bin to the next. Our PPL reconstruction of the NG15 signal relies on individual PPL models with a fixed number of internal nodes (i.e., constant power law, broken power law, doubly broken power law, etc.), which are ultimately combined in a Bayesian model average. The data products resulting from our analysis provide the basis for fast refits of spectral GWB models.
Author(s): Agazie G, Anumarlapudi A, Archibald AM, Arzoumanian Z, Baier JG, Baker PT, Becsy B, Bhoonah A, Blecha L, Brazier A, Brook PR, Burke-Spolaor S, Burnette R, Case R, 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, Ferrara EC, Fiore W, Fonseca E, Freedman GE, Gardiner EC, Garver-Daniels N, Gentile PA, Gersbach KA, Glaser J, Gomez-Cortes BD, Good DC, Gultekin K, Harris CJ, 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 TJW, Lewandowska N, Leys M, Liu T, Lorimer DR, Luo J, Lynch RS, Ma C-P, Madison DR, Matt C, 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, Petrov P, Pol NS, Radovan HA, Ransom SM, Ray PS, Romano JD, Runnoe JC, Saffer A, Sardesai SC, Schmiedekamp A, Schmiedekamp C, Schmitz K, Shapiro-Albert BJ, Siemens X, Simon J, Fiscella SVS, Stairs IH, Stinebring DR, Stovall K, Susobhanan A, Swiggum JK, Taylor J, Taylor SR, Thompson MS, Turner JE, Vallisneri M, van Haasteren R, Vigeland SJ, Wahl HM, Wang S, Wilson KP, Witt CA, Wright D, Young O
Publication type: Article
Publication status: Published
Journal: Astrophysical Journal Letters
Year: 2026
Volume: 1004
Issue: 2
Print publication date: 20/06/2026
Online publication date: 16/06/2026
Acceptance date: 19/05/2026
Date deposited: 01/07/2026
ISSN (print): 2041-8205
ISSN (electronic): 2041-8213
Publisher: American Astronomical Society
URL: https://doi.org/10.3847/2041-8213/ae7086
DOI: 10.3847/2041-8213/ae7086
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