The Nanograv Nine-Year Data Set: Limits on the Isotropic Stochastic Gravitational Wave Background

Abstract

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 1.5 10 gw 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, W <´-f h 4.2 10 gw 2 1 0 ( ) . Our limit on the cosmic string GWB, W <´-f h 2.2 10 gw 2 1 0 ( ) , translates to a conservative limit on the cosmic string tension with m <´-G 3.3 10 8 , a factor of four better than the joint Planck and high-lcosmic microwave background data from other experiments.

Production of high stellar-mass primordial black holes in trapped inflation

“…The peak is associated with the production of about 14M ⊙ PBHs. The figure also shows the current upper limits on gravitational wave background made by the pulsar timing array experiments EPTA [43], NANOGrav [44], and PPTA [45], and the projected sensitivity of SKA radio telescope [46]. The gravitational waves associated with the production of 14M ⊙ PBHs are about an order of magnitude below the current pulsar timing array sensitivity.…”

Evolution of massive black hole binaries in rotating stellar nuclei: Implications for gravitational wave detection

“…Simple evolutionary models suggest f bend ≈ 10 −9 Hz [33], a frequency regime that is beginning to be probed by PTAs. Analysis of pulsar timing data by three groups has so far succeeded in placing only upper limits on A yr : 3.0 × 10 −15 (EPTA [8]); 1.0 × 10 −15 (PPTA [37]); and 1.5 × 10 −15 (NANOGrav [1]). These values are generally interpreted as being "in tension with" the predictions of some theoretical models; for instance, McWilliams et al [20] predict A yr ≈ 10 −14.5 .…”

“…At these lower frequencies, f f yr , the characteristic strain is expected to differ from the prediction of Eq. (1). The semimajor axis of a binary SBH with orbital period P is a = 1.0 × 10 −2 M 12 10 8 M 1/3 P 10 yr …”

Probing the properties of the pulsar wind via studying the dispersive effects in the pulses from the pulsar companion in a double neutron-star binary system

“…A satisfactory theory of magnetosphere is essential for modeling pulsar braking, which is the main cause of pulsar timing noises (Hobbs et al 2010). Reducing the timing noise is the major stream of efforts in pulsar timing, so as to unveil small signals such as gravitational waves (Arzoumanian et al 2016;Babak et al 2016;Yi & Zhang 2016;Zhu et al 2016). Observational constrains on the density and velocity distribution of e ± in pulsar wind can test and select among these theories of magnetosphere;…”