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 pulsars using the Green Bank and Arecibo radio
telescopes. In this work we use a subset of 17 pulsars timed for a span of
roughly five years (2005--2010). We analyze these data using standard pulsar
timing 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 waves from individual supermassive black
hole binaries in circular orbits using 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 pulsar dataset 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
distance to such gravitational wave sources 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 wave sources near our best timed
pulsars in the sky, the sensitivity of the pulsar timing array is 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 hole binaries.
