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The NANOGrav Nine-year Data Set: Limits on the Isotropic Stochastic Gravitational Wave Background
Zaven Arzoumanian, Adam Brazier, Sarah Burke-Spolaor, Sydney Chamberlin, Shami Chatterjee, Brian Christy, Jim Cordes, Neil Cornish, Paul Demorest, Xihao Deng, Tim Dolch, Justin Ellis, Rob Ferdman, Emmanuel Fonseca, Nate Garver-Daniels, Fredrick Jenet, Glenn Jones, Vicky Kaspi, Michael Koop, Michael Lam, Joseph Lazio, Lina Levin, Andrea Lommen, Duncan Lorimer, Jin Luo, Ryan Lynch, Dustin Madison, Maura McLaughlin, Sean McWilliams, Chiara Mingarelli, David Nice, Nipuni Palliyaguru, Tim Pennucci, Scott Ransom, Laura Sampson, Sotiris Sanidas, Alberto Sesana, Xavier Siemens, Joseph Simon, Ingrid Stairs, Dan Stinebring, Kevin Stovall, Joseph Swiggum, Stephen Taylor, Michele Vallisneri, Rutger van Haasteren, Yan Wang, Weiwei Zhu
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We compute upper limits on the nanohertz-frequency isotropic stochastic gravitational wave background (GWB) using the 9-year data release from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration. We set upper limits for a GWB from supermassive black hole binaries under power law, broken power law, and free spectral coefficient GW spectrum models. We place a 95\% upper limit on the strain amplitude (at a frequency of yr⁻¹) in the power law model of A_(gw) < 1.5×10⁻¹⁵. For a broken power law model, we place priors on the strain amplitude derived from simulations of Sesana (2013) and McWilliams et al. (2014). We find that the data favor a broken power law to a pure power law with odds ratios of 22 and 2.2 to one for the McWilliams and Sesana prior models, respectively. The McWilliams model is essentially ruled out by the data, and the Sesana model is in tension with the data under the assumption of a pure power law. 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. We then place the most stringent limits so far on the energy density of relic GWs, Ω_(gw)(f) h² < 4.2 ×10⁻¹⁰, yielding a limit on the Hubble parameter during inflation of H_*=1.6×10⁻² m_(Pl), where m_(Pl) is the Planck mass. Our limit on the cosmic string GWB, Ω_(gw)(f) h² < 2.2 ×10⁻¹⁰, translates to a conservative limit of Gμ<3.3×10⁻⁸ - a factor of 4 better than the joint Planck and high-l CMB data from other experiments.
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