The NANOGrav 11-year Data Set: Pulsar-timing Constraints On The Stochastic Gravitational-wave Background

Z. Arzoumanian, P. T. Baker, A. Brazier, S. Burke-Spolaor, S. J. Chamberlin, S. Chatterjee, B. Christy, J. M. Cordes, N. J. Cornish, F. Crawford, H. Thankful Cromartie, K. Crowter, M. DeCesar, P. B. Demorest, T. Dolch, J. A. Ellis, R. D. Ferdman, E. Ferrara, W. M. Folkner, E. Fonseca, N. Garver-Daniels, P. A. Gentile, R. Haas, J. S. Hazboun, E. A. Huerta, K. Islo, F. Jenet, G. Jones, M. L. Jones, D. L. Kaplan, V. M. Kaspi, M. T. Lam, T. J. W. Lazio, L. Levin, A. N. Lommen, D. R. Lorimer, J. Luo, R. S. Lynch, D. R. Madison, M. A. McLaughlin, S. T. McWilliams, C. M. F. Mingarelli, C. Ng, D. J. Nice, R. S. Park, T. T. Pennucci, N. S. Pol, S. M. Ransom, P. S. Ray, A. Rasskazov, X. Siemens, J. Simon, R. Spiewak, I. H. Stairs, D. R. Stinebring, K. Stovall, J. Swiggum, S. R. Taylor, M. Vallisneri, S. Vigeland, W. W. Zhu

Published 2018-01-08Version 1

We search for an isotropic stochastic gravitational-wave background (GWB) in the newly released $11$-year dataset from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). While we find no significant evidence for a GWB, we place constraints on a GWB from a population of supermassive black-hole binaries, from cosmic strings, and from a primordial GWB. For the first time, we find that the GWB upper limits and detection statistics are sensitive to the Solar System ephemeris (SSE) model used, and that SSE errors can mimic a GWB signal. To mitigate this effect, we developed and implemented a novel approach that bridges systematic SSE differences, producing the first PTA constraints that are robust against SSE uncertainties. We place a $95\%$ upper limit on the GW strain amplitude of $A_\mathrm{GWB}<1.45\times 10^{-15}$ at a frequency of $f=1$ yr$^{-1}$ for a fiducial $f^{-2/3}$ power-law spectrum, and with inter-pulsar correlations modeled. This is a factor of $\sim 2$ improvement over the NANOGrav $9$-year limit, calculated using the same procedure. Previous PTA upper limits on the GWB will need revision in light of SSE systematic uncertainties. We also characterize the combined influence of the mass-density of stars in galactic cores, the eccentricity of binaries at formation, and the relation between the mass of the central supermassive black hole and the galactic bulge (the $M_\mathrm{BH}-M_\mathrm{bulge}$ relation). We constrain cosmic-string tension on the basis of recent simulations, yielding an SSE-marginalized 95\% upper limit on the cosmic string tension of $G\mu < 5.3\times 10^{-11}$---a factor of $\sim 2$ better than the NANOGrav $9$-year constraints. We then use our new Bayesian SSE model to limit the energy density of primordial GWBs, corresponding to $\Omega_\mathrm{GWB}(f)h^2<3.4 \times 10^{-10}$ for a radiation-dominated inflationary era. [ABRIDGED]