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

TL;DR

This study uses pulsar timing data to search for continuous gravitational waves from supermassive black hole binaries, setting the most stringent upper limits to date on their strain amplitude and distance, but finds no evidence of detection.

Contribution

The paper provides the first comprehensive upper limits on gravitational wave strain and source distance from supermassive black hole binaries using NANOGrav data.

Findings

No evidence for continuous gravitational waves detected.

Placed a 95% upper limit on strain amplitude of ~3.8×10^{-14} at 10 nHz.

Set lower limits on source distance, d_L ≥ 425 Mpc at 10 nHz.

Abstract

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.