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


Agazie G., Anumarlapudi A., Archibald A. M., Arzoumanian Z., Baier J. G., Baker P. T., ...More

Astrophysical Journal Letters, vol.978, no.2, 2025 (SCI-Expanded) identifier

  • Publication Type: Article / Article
  • Volume: 978 Issue: 2
  • Publication Date: 2025
  • Doi Number: 10.3847/2041-8213/ad99d3
  • Journal Name: Astrophysical Journal Letters
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus, Aerospace Database, INSPEC, Directory of Open Access Journals, Nature Index
  • Middle East Technical University Affiliated: Yes

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.