THE NANOGRAV NINE-YEAR DATA SET: LIMITS ON THE ISOTROPIC STOCHASTIC GRAVITATIONAL WAVE BACKGROUND

We compute upper limits on the nanohertz-frequency isotropic stochastic gravitational wave background (GWB) using the 9 year data set from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration. Well-tested Bayesian techniques are used to set upper limits on the dimensionless strain amplitude (at a frequency of 1 yr^(−1) for a GWB from supermassive black hole binaries of A_(gw) < 1.5 x 10^(-15). We also parameterize the GWB spectrum with a broken power-law model by placing priors on the strain amplitude derived from simulations of Sesana and McWilliams et al. Using Bayesian model selection we find that the data favor a broken power law to a pure power law with odds ratios of 2.2 and 22 to one for the Sesana and McWilliams prior models, respectively. Using the broken power-law analysis we construct posterior distributions on environmental factors that drive the binary to the GW-driven regime including the stellar mass density for stellar-scattering, mass accretion rate for circumbinary disk interaction, and orbital eccentricity for eccentric binaries, marking the first time that the shape of the GWB spectrum has been used to make astrophysical inferences. Returning to a power-law model, we place stringent limits on the energy density of relic GWs, Ω_(gw)(f)h^2 < 4.2 x 10^(-10). Our limit on the cosmic string GWB, Ω_(gw)(f)h^2 < 2.2 x 10^(-10), translates to a conservative limit on the cosmic string tension with Gµ < 3.3 x 10^(-8), a factor of four better than the joint Planck and high-l cosmic microwave background data from other experiments.

Yan Wang | Zaven Arzoumanian | Rutger van Haasteren | Kevin Stovall | Joseph Simon | Weiwei Zhu | Adam Brazier | Shami Chatterjee | Maura McLaughlin | Michele Vallisneri | Joseph Lazio | Sarah Burke-Spolaor | Scott Ransom | Xavier Siemens | Emmanuel Fonseca | Alberto Sesana | Neil Cornish | Sean McWilliams | D. Stinebring | J. Luo | S. McWilliams | X. Siemens | S. Chamberlin | N. Cornish | R. Lynch | L. Sampson | S. Burke-Spolaor | J. Cordes | Z. Arzoumanian | M. Mclaughlin | J. Swiggum | D. Lorimer | S. Ransom | A. Brazier | D. Nice | J. Ellis | F. Jenet | V. Kaspi | I. Stairs | K. Stovall | S. Chatterjee | T. Lazio | P. Demorest | M. Gonzalez | A. Sesana | M. Vallisneri | Y. Wang | M. Lam | B. Christy | C. Mingarelli | R. Haasteren | N. Palliyaguru | I. Stairs | T. Pennucci | L. Levin | R. Ferdman | S. Sanidas | E. Fonseca | M. Jones | A. Lommen | D. Madison | T. Dolch | N. Garver-Daniels | J. Simon | M. Koop | K. Crowter | Glenn Jones | Ryan Lynch | Ingrid Stairs | Dustin Madison | Chiara Mingarelli | Paul Demorest | Duncan Lorimer | Tim Pennucci | Dan Stinebring | Nipuni Palliyaguru | Stephen Taylor | Sydney Chamberlin | Brian Christy | Jim Cordes | Xihao Deng | Tim Dolch | Justin Ellis | Rob Ferdman | Nate Garver-Daniels | Fredrick Jenet | Vicky Kaspi | Michael Koop | Michael Lam | Lina Levin | Andrea Lommen | Jin Luo | David Nice | Laura Sampson | Sotiris Sanidas | Joseph Swiggum | X. Deng | J. Luo | S. Taylor | J. Cordes | Y. Wang | W. W. Zhu | G. Jones | R. V. Haasteren | G. Jones | S. Chatterjee | V. Kaspi | L. Levin | S. Taylor | W. Zhu | B. Christy | X. Deng | S. Chatterjee

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