{
  "id": "2310.11238",
  "version": "v1",
  "published": "2023-10-17T13:08:21.000Z",
  "updated": "2023-10-17T13:08:21.000Z",
  "title": "Search for Non-Tensorial Gravitational-Wave Backgrounds in the NANOGrav 15-Year Data Set",
  "authors": [
    "Zu-Cheng Chen",
    "Yu-Mei Wu",
    "Yan-Chen Bi",
    "Qing-Guo Huang"
  ],
  "comment": "10 pages, 5 figures, 2 tables",
  "categories": [
    "astro-ph.CO",
    "gr-qc",
    "hep-ph",
    "hep-th"
  ],
  "abstract": "The recent detection of a stochastic signal in the NANOGrav 15-year data set has aroused great interest in uncovering its origin. However, the evidence for the Hellings-Downs correlations, a key signature of the gravitational-wave background (GWB) predicted by general relativity, remains inconclusive. In this paper, we search for an isotropic non-tensorial GWB, allowed by general metric theories of gravity, in the NANOGrav 15-year data set. Our analysis reveals a Bayes factor of approximately 2.5, comparing the quadrupolar (tensor transverse, TT) correlations to the scalar transverse (ST) correlations, suggesting that the ST correlations provide a comparable explanation for the observed stochastic signal in the NANOGrav data. We obtain the median and the $90\\%$ equal-tail amplitudes as $\\mathcal{A}_\\mathrm{ST} = 7.8^{+5.1}_{-3.5} \\times 10^{-15}$ at the frequency of 1/year. Furthermore, we find that the vector longitudinal (VL) and scalar longitudinal (SL) correlations are weakly and strongly disfavoured by data, respectively, yielding upper limits on the amplitudes: $\\mathcal{A}_\\mathrm{VL}^{95\\%} \\lesssim 1.7 \\times 10^{-15}$ and $\\mathcal{A}_\\mathrm{SL}^{95\\%} \\lesssim 7.4 \\times 10^{-17}$. Lastly, we fit the NANOGrav data with a general transverse (GT) correlations parameterized by a free parameter $\\alpha$. Our analysis yields $\\alpha=1.74^{+1.18}_{-1.41}$, thus excluding both the TT ($\\alpha=3$) and ST ($\\alpha=0$) models at the $90\\%$ confidence level.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2023-10-17T13:08:21.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "data set",
      "non-tensorial gravitational-wave backgrounds",
      "stochastic signal",
      "nanograv data",
      "general metric theories"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 10,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}