The NANOGrav 11 yr Data Set: Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries

Observations indicate that nearly all galaxies contain supermassive black holes at their centers. When galaxies merge, their component black holes form SMBH binaries (SMBHBs), which emit low-frequency gravitational waves (GWs) that can be detected by pulsar timing arrays. We have searched the North American Nanohertz Observatory for Gravitational Waves 11 yr data set for GWs from individual SMBHBs in circular orbits. As we did not find strong evidence for GWs in our data, we placed 95% upper limits on the strength of GWs from such sources. At {f}_gw} = 8 nHz, we placed a sky-averaged upper limit of h ₀ < 7.3(3) × 10^-15. We also developed a technique to determine the significance of a particular signal in each pulsar using “dropout” parameters as a way of identifying spurious signals. From these upper limits, we ruled out SMBHBs emitting GWs with {f}_gw} = 8 nHz within 120 Mpc for { \mathcal M }={10}⁹ {M}_⊙ , and within 5.5 Gpc for { \mathcal M }={10}¹⁰ {M}_⊙ at our most sensitive sky location. We also determined that there are no SMBHBs with { \mathcal M }> 1.6× {10}⁹ {M}_⊙ emitting GWs with {f}_gw} = 2.8-317.8 nHz in the Virgo Cluster. Finally, we compared our strain upper limits to simulated populations of SMBHBs, based on galaxies in the Two Micron All-Sky Survey and merger rates from the Illustris cosmological simulation project, and found that only 34 out of 75,000 realizations of the local universe contained a detectable source.