The NANOGrav 12.5-year Data Set: Search For An Isotropic Stochastic Gravitational-Wave Background
Creators: name: Arzoumanian, Zaven
name: Baker, Paul T.
name: Blumer, Harsha
name: Becsy, Bence
name: Brazier, Adam
name: Brook, Paul R.
name: Burke-Spolaor, Sarah
name: Chatterjee, Shami
name: Chen, Siyuan
name: Cordes, James M.
name: Cornish, Neil J.
name: Crawford, Fronefield
name: Cromartie, H. Thankful
name: DeCesar, Megan E.
name: Demorest, Paul B.
name: Dolch, Timothy
name: Ellis, Justin A.
name: Ferrara, Elizabeth C.
name: Fiore, William
name: Fonseca, Emmanuel
name: Garver-Daniels, Nathan
name: Gentile, Peter A.
name: Good, Deborah C.
name: Hazboun, Jeffrey S.
name: Holgado, A. Miguel
name: Islo, Kristina
name: Jennings, Ross J.
name: Jones, Megan L.
name: Kaiser, Andrew R.
name: Kaplan, David L.
name: Kelley, Luke Zoltan
name: Key, Joey Shapiro
name: Laal, Nima
name: Lam, Michael T.
name: Lazio, T. Joseph W.
name: Lorimer, Duncan R.
name: Luo, Jing
name: Lynch, Ryan S.
name: Madison, Dustin R.
name: McLaughlin, Maura A.
name: Mingarelli, Chiara M. F.
name: Ng, Cherry
name: Nice, David J.
name: Pennucci, Timothy T.
name: Pol, Nihan S.
name: Ransom, Scott M.
name: Ray, Paul S.
name: Shapiro-Albert, Brent J.
name: Siemens, Xavier
name: Simon, Joseph
name: Spiewak, Renee
name: Stairs, Ingrid H.
name: Stinebring, Daniel R.
name: Stovall, Kevin
name: Sun, Jerry P.
name: Swiggum, Joseph K.
name: Taylor, Stephen R.
name: Turner, Jacob E.
name: Vallisneri, Michele
name: Vigeland, Sarah J.
name: Witt, Caitlin A.
Publisher: name: arXiv
Year: 2020
Subjects: subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: Astrophysics of Galaxies (astro-ph.GA)
subject: General Relativity and Quantum Cosmology (gr-qc)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We search for an isotropic stochastic gravitational-wave background (GWB) in the $12.5$-year pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. Our analysis finds strong evidence of a stochastic process, modeled as a power-law, with common amplitude and spectral slope across pulsars. The Bayesian posterior of the amplitude for an $f^{-2/3}$ power-law spectrum, expressed as the characteristic GW strain, has median $1.92 \times 10^{-15}$ and $5\%$--$95\%$ quantiles of $1.37$--$2.67 \times 10^{-15}$ at a reference frequency of $f_\mathrm{yr} = 1 ~\mathrm{yr}^{-1}$. The Bayes factor in favor of the common-spectrum process versus independent red-noise processes in each pulsar exceeds $10,000$. However, we find no statistically significant evidence that this process has quadrupolar spatial correlations, which we would consider necessary to claim a GWB detection consistent with general relativity. We find that the process has neither monopolar nor dipolar correlations, which may arise from, for example, reference clock or solar system ephemeris systematics, respectively. The amplitude posterior has significant support above previously reported upper limits; we explain this in terms of the Bayesian priors assumed for intrinsic pulsar red noise. We examine potential implications for the supermassive black hole binary population under the hypothesis that the signal is indeed astrophysical in nature.
description: 25 pages, 14 figures, 5 tables, 3 appendices. Published in The Astrophysical Journal Letters. Please send any comments/questions to Joseph Simon (joe.simon@nanograv.org). Jupyter notebook tutorials and some MCMC chain files are available at https://github.com/nanograv/12p5yr_stochastic_analysis