New Nanohertz Gravitational Wave Limits
New upper limits have been computed on the nanohertz-frequency isotropic stochastic gravitational wave background (GWB) using the nine-year data set from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration. Well-tested Bayesian techniques set upper limits on the dimensionless strain amplitude for a GWB from supermassive black hole binaries of Agw < 1.5 x 10-15. The Collaboration parameterizes 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, the NANOGrav 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, posterior distributions are constructed on environmental factors that drive the binary to the gravitational wave-driven regime, including the stellar mass density for stellar-scattering, mass accretion rate for circum-binary 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, stringent limits are placed on the energy density of relic GWs, Ωgw(f)h2 < 4.2 x 10-10. The limit on the cosmic string GWB, Ωgw(f)h2 < 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.
Science Team: Z. Arzoumanian (NASA-Goddard), A. Brazier (Cornell), S. Burke-Spolaor (NRAO), S. J. Chamberlin (Penn State), S. Chatterjee (Cornell), B. Christy (Franklin & Marshall), J. M. Cordes (Cornell), N. J. Cornish (Montana State), K. Crowter (British Columbia), P. B. Demorest (NRAO), X. Deng (Penn State), T. Dolch (Cornell, Hillsdale), J. A. Ellis (JPL), R. D. Ferdman (McGill), E. Fonseca (British Columbia), N. Garver-Daniels (West Virginia), M. E. Gonzalez (British Columbia, Vancouver), F. Jenet (Texas-Brownsville), G. Jones (Columbia), M. L. Jones (West Virginia), V. M. Kaspi (McGill), M. Koop (Penn State), M. T. Lam (Cornell), T. J. W. Lazio (JPL), L. Levin (West Virginia), A. N. Lommen (Franklin & Marshall), D. R. Lorimer (West Virginia), J. Luo (Texas-Brownsville), R. S. Lynch (NRAO), D. R. Madison (Cornell, NRAO), M. A. McLaughlin (West Virginia), S. T. McWilliams (West Virginia), C. M. F. Mingarelli (TAPIR, MPIfR), D. J. Nice (Lafayette), N. Palliyaguru (West Virginia), T. T. Pennucci (Virginia), S. M. Ransom (NRAO), L. Sampson (Montana State), S. A. Sanidas (Amsterdam, Jodrell Bank), A. Sesana (Birmingham), X. Siemens (Wisconsin-Milwaukee), J. Simon (Wisconsin-Milwaukee), I. H. Stairs (British Columbia), D. R. Stinebring (Oberlin), K. Stovall (New Mexico), J. Swiggum (West Virginia), S. R. Taylor (JPL), M. Vallisneri (JPL), R. van Haasteren (JPL), Y. Wang (Huazhong), and W. W. Zhu (British Columbia, MPIfR).
Publication: The NANOGrav Nine-Year Data Set: Limits on the Isotropic Stochastic Gravitational Wave Background, 2016 Astrophysical Journal, 821, 13.
