LIMITS ON THE STOCHASTIC GRAVITATIONAL WAVE BACKGROUND FROM THE NORTH AMERICAN NANOHERTZ OBSERVATORY FOR GRAVITATIONAL WAVES

Abstract

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