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© The Author(s), under exclusive licence to Springer Nature Limited 2026.The detection of a stochastic gravitational wave background by pulsar-timing arrays indicates the presence of a population of supermassive black hole binaries. Although the observed spectrum generally matches predictions for orbital evolution driven by gravitational-wave emission in circular orbits, there is a preference for a spectral turnover at the lowest observed frequencies, which may point to substantial hardening during a transition from early environmental influences to later stages dominated by emission. In the vicinity of these binaries, the ejection of stars or dark matter particles through gravitational three-body slingshots efficiently extracts orbital energy, leading to a low-frequency turnover in the spectrum. Here we model how the gravitational-wave spectrum depends on the initial inner galactic profile before scouring by binary ejections while accounting for a range of initial binary eccentricities. By analysing the NANOGrav 15-year data, we find that a parsec-scale galactic-centre density of around 106 M⊙ pc−3 is favoured across most of the parameter space, thus shedding light on the environmental effects that shape black hole evolution and the combined matter density near galaxy centres.
Author(s): Chen Y, Daniel M, D'Orazio DJ, Fan X, Mitridate A, Sagunski L, Xue X, Young O, Wright D, Witt CA, Wahl HM, Vigeland SJ, van Haasteren R, Vallisneri M, Unal C, Turner JE, Taylor SR, Taylor J, Swiggum JK, Susobhanan A, Stovall K, Stinebring DR, Stairs IH, Sosa Fiscella SV, Siwek MS, Simon J, Siemens X, Shapiro-Albert BJ, Schmitz K, Schmiedekamp C, Schmiedekamp A, Sardesai SC, Saffer A, Runnoe JC, Romano JD, Ray PS, Ransom SM, Radovan HA, Pol NS, Petrov P, Perera BBP, Pennucci TT, Olum KD, Ocker SK, Nice DJ, Ng C, Mingarelli CMF, Meyers PM, Meyers BW, McMann N, McLaughlin MA, McKee JW, McEwen A, Madison DR, Ma C-P, Lynch RS, Luo J, Lorimer DR, Liu T, Lewandowska N, Lazio TJW, Larsen B, Lamb WG, Lam MT, Laal N, Key JS, Kerr M, Kelley LZ, Kaplan DL, Jones ML, Johnson AD, Jennings RJ, Hazboun JS, Gultekin K, Good DC, Glaser J, Gersbach KA, Gentile PA, Garver-Daniels N, Gardiner EC, Freedman GE, Fonseca E, Fiore W, Ferrara EC, Dolch T, Dey L, Deng H, Demorest PB, DeCesar ME, Crowter K, Cromartie HT, Crawford F, Cornish NJ, Cordes JM, Cohen T, Chatterjee S, Charisi M, Casey-Clyde JA, Burnette R, Burke-Spolaor S, Brook PR, Brazier A, Blecha L, Becsy B, Baker PT, Baier JG, Arzoumanian Z, Archibald AM, Anumarlapudi A, Agazie G
Publication type: Article
Publication status: Published
Journal: Nature Astronomy
Year: 2026
Pages: epub ahead of print
Online publication date: 05/02/2026
Acceptance date: 06/01/2026
ISSN (electronic): 2397-3366
Publisher: Nature Research
URL: https://doi.org/10.1038/s41550-026-02782-0
DOI: 10.1038/s41550-026-02782-0
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