Limits on the Stochastic Gravitational Wave Background from the North American Nanohertz Observatory for Gravitational Waves

TL;DR

This paper analyzes five years of pulsar timing data from NANOGrav to set upper limits on the nanohertz gravitational wave background, achieving high-precision residuals and developing methods for detecting correlated signals.

Contribution

It introduces new analysis methods for pulsar timing residuals that account for timing model effects and applies them to set the first stringent upper limits on the stochastic gravitational wave background at nanohertz frequencies.

Findings

Achieved sub-50 nanosecond timing residuals in pulsar data.

Set an upper limit of h_c < 7x10^-15 on the gravitational wave background.

Identified the two best pulsars as dominant in the limit setting.

Abstract

We present an analysis of high-precision pulsar timing data taken as part of the North American Nanohertz Observatory for Gravitational waves (NANOGrav) project. We have observed 17 pulsars for a span of roughly five years using the Green Bank and Arecibo radio telescopes. We analyze these data using standard pulsar timing models, with the addition of time-variable dispersion measure and frequency-variable pulse shape terms. Sub-microsecond timing residuals are obtained in nearly all cases, and the best root-mean-square timing residuals in this set are ~30-50 ns. We present methods for analyzing post-fit timing residuals for the presence of a gravitational wave signal with a specified spectral shape. These optimally take into account the timing fluctuation power removed by the model fit, and can be applied to either data from a single pulsar, or to a set of pulsars to detect a correlated signal. We apply these methods to our dataset to set an upper limit on the strength of the nHz-frequency stochastic supermassive black hole gravitational wave background of h_c (1 yr^-1) < 7x10^-15 (95%). This result is dominated by the timing of the two best pulsars in the set, PSRs J1713+0747 and J1909-3744.