Abstract
We compute upper limits on the nanohertzfrequency isotropic stochastic gravitational wave background (GWB) using the 9 year data set from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration. Welltested 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 powerlaw 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 powerlaw analysis we construct posterior distributions on environmental factors that drive the binary to the GWdriven regime including the stellar mass density for stellarscattering, 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 powerlaw model, we place stringent limits on the energy density of relic GWs, Omega(gw) (f)h(2) <4.2 x 10(10). Our limit on the cosmic string GWB, Omega(gw) (f)h(2) <2.2 x 10(10), translates to a conservative limit on the cosmic string tension with G mu <3.3 x 10(8), a factor of four better than the joint Planck and highl cosmic microwave background data from other experiments.
Original language  English 

Article number  13 
Number of pages  23 
Journal  Astrophysical Journal 
Volume  821 
Issue number  1 
DOIs  
Publication status  Published  10 Apr 2016 
Externally published  Yes 
Profiles

Robert Ferdman
 School of Engineering, Mathematics and Physics  Associate Professor in Physics
Person: Academic, Teaching & Research