NANOGrav Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries in Circular Orbits

The North American Nanohertz Observatory for Gravitational Waves(NANOGrav) project currently observes 43 pulsarsusing the Green Bank and Arecibo radio telescopes. In this work we use a subset of 17 pulsarstimed for a span of roughly five years (2005--2010). We analyze these data using standard pulsartiming models, with the addition of time-variable dispersion measure and frequency-variable pulse shape terms. Within the timing data, we perform a search for continuous gravitational wavesfrom individual supermassive black holebinaries in circular orbitsusing robust frequentist and Bayesian techniques. We find that there is no evidence for the presence of a detectable continuous gravitational wave; however, we can use these data to place the most constraining upper limits to date on the strength of such gravitational waves. Using the full 17 pulsardataset we place a 95% upper limit on the sky-averaged strain amplitude of $h_0\lesssim 3.8\times 10^{-14}$ at a frequency of 10 nHz. Furthermore, we place 95% \emph{all sky} lower limits on the luminosity distanceto such gravitational wavesources finding that the $d_L \gtrsim 425$ Mpc for sources at a frequency of 10 nHz and chirp mass $10^{10}{\rm M}_{\odot}$. We find that for gravitational wavesources near our best timed pulsarsin the sky, the sensitivity of the pulsar timing arrayis increased by a factor of $\sim$4 over the sky-averaged sensitivity. Finally we place limits on the coalescence rate of the most massive supermassive black holebinaries.