The NANOGrav 15-year Data Set: Evidence for a Gravitational-Wave Background
Creators: name: Agazie, Gabriella
name: Anumarlapudi, Akash
name: Archibald, Anne M.
name: Arzoumanian, Zaven
name: Baker, Paul T.
name: Becsy, Bence
name: Blecha, Laura
name: Brazier, Adam
name: Brook, Paul R.
name: Burke-Spolaor, Sarah
name: Burnette, Rand
name: Case, Robin
name: Charisi, Maria
name: Chatterjee, Shami
name: Chatziioannou, Katerina
name: Cheeseboro, Belinda D.
name: Chen, Siyuan
name: Cohen, Tyler
name: Cordes, James M.
name: Cornish, Neil J.
name: Crawford, Fronefield
name: Cromartie, H. Thankful
name: Crowter, Kathryn
name: Cutler, Curt J.
name: DeCesar, Megan E.
name: DeGan, Dallas
name: Demorest, Paul B.
name: Deng, Heling
name: Dolch, Timothy
name: Drachler, Brendan
name: Ellis, Justin A.
name: Ferrara, Elizabeth C.
name: Fiore, William
name: Fonseca, Emmanuel
name: Freedman, Gabriel E.
name: Garver-Daniels, Nate
name: Gentile, Peter A.
name: Gersbach, Kyle A.
name: Glaser, Joseph
name: Good, Deborah C.
name: Gultekin, Kayhan
name: Hazboun, Jeffrey S.
name: Hourihane, Sophie
name: Islo, Kristina
name: Jennings, Ross J.
name: Johnson, Aaron D.
name: Jones, Megan L.
name: Kaiser, Andrew R.
name: Kaplan, David L.
name: Kelley, Luke Zoltan
name: Kerr, Matthew
name: Key, Joey S.
name: Klein, Tonia C.
name: Laal, Nima
name: Lam, Michael T.
name: Lamb, William G.
name: Lazio, T. Joseph W.
name: Lewandowska, Natalia
name: Littenberg, Tyson B.
name: Liu, Tingting
name: Lommen, Andrea
name: Lorimer, Duncan R.
name: Luo, Jing
name: Lynch, Ryan S.
name: Ma, Chung-Pei
name: Madison, Dustin R.
name: Mattson, Margaret A.
name: McEwen, Alexander
name: McKee, James W.
name: McLaughlin, Maura A.
name: McMann, Natasha
name: Meyers, Bradley W.
name: Meyers, Patrick M.
name: Mingarelli, Chiara M. F.
name: Mitridate, Andrea
name: Natarajan, Priyamvada
name: Ng, Cherry
name: Nice, David J.
name: Ocker, Stella Koch
name: Olum, Ken D.
name: Pennucci, Timothy T.
name: Perera, Benetge B. P.
name: Petrov, Polina
name: Pol, Nihan S.
name: Radovan, Henri A.
name: Ransom, Scott M.
name: Ray, Paul S.
name: Romano, Joseph D.
name: Sardesai, Shashwat C.
name: Schmiedekamp, Ann
name: Schmiedekamp, Carl
name: Schmitz, Kai
name: Schult, Levi
name: Shapiro-Albert, Brent J.
name: Siemens, Xavier
name: Simon, Joseph
name: Siwek, Magdalena S.
name: Stairs, Ingrid H.
name: Stinebring, Daniel R.
name: Stovall, Kevin
name: Sun, Jerry P.
name: Susobhanan, Abhimanyu
name: Swiggum, Joseph K.
name: Taylor, Jacob
name: Taylor, Stephen R.
name: Turner, Jacob E.
name: Unal, Caner
name: Vallisneri, Michele
name: van Haasteren, Rutger
name: Vigeland, Sarah J.
name: Wahl, Haley M.
name: Wang, Qiaohong
name: Witt, Caitlin A.
name: Young, Olivia
Publisher: name: arXiv
Year: 2023
Subjects: subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: General Relativity and Quantum Cosmology (gr-qc)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We report multiple lines of evidence for a stochastic signal that is correlated among 67 pulsars from the 15-year pulsar-timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. The correlations follow the Hellings-Downs pattern expected for a stochastic gravitational-wave background. The presence of such a gravitational-wave background with a power-law-spectrum is favored over a model with only independent pulsar noises with a Bayes factor in excess of $10^{14}$, and this same model is favored over an uncorrelated common power-law-spectrum model with Bayes factors of 200-1000, depending on spectral modeling choices. We have built a statistical background distribution for these latter Bayes factors using a method that removes inter-pulsar correlations from our data set, finding $p = 10^{-3}$ (approx. $3σ$) for the observed Bayes factors in the null no-correlation scenario. A frequentist test statistic built directly as a weighted sum of inter-pulsar correlations yields $p = 5 \times 10^{-5} - 1.9 \times 10^{-4}$ (approx. $3.5 - 4σ$). Assuming a fiducial $f^{-2/3}$ characteristic-strain spectrum, as appropriate for an ensemble of binary supermassive black-hole inspirals, the strain amplitude is $2.4^{+0.7}_{-0.6} \times 10^{-15}$ (median + 90% credible interval) at a reference frequency of 1/(1 yr). The inferred gravitational-wave background amplitude and spectrum are consistent with astrophysical expectations for a signal from a population of supermassive black-hole binaries, although more exotic cosmological and astrophysical sources cannot be excluded. The observation of Hellings-Downs correlations points to the gravitational-wave origin of this signal.
description: 30 pages, 18 figures. Published in Astrophysical Journal Letters as part of Focus on NANOGrav's 15-year Data Set and the Gravitational Wave Background. For questions or comments, please email comments@nanograv.org